The King George Brothel in Berlin opens at 4 PM, Monday through to Sunday, excluding the Christmas Holidays. For €99 (£78) you can stay there until dawn, when the first street cleaners are passing, and drink as much as you like and have sex as much as you like with Klaudia, Katja, Petronella, Alina, Barby and whichever of the 27 staff members are perched on the velvet stools or the leather booths beneath the many red lights next to the bar.

The King George is Germany's first flat-rate brothel. It's the sex industry's answer to the global recession. In Berlin there are about half a dozen of them. The brothels themselves prefer calling it "all-inclusive" rather than flat-rate. As owner Sascha Erben says, "this is sex after all, it's not text messaging or long distance calls."

Alina works the door. She's wearing this pink tube dress. When she gets up from her chair, the dress zips up over her ass until she jigs it down again with her fingers. The rest of the girls wear the same dress in different grades of pink. It's like a house uniform. Zipping up and down, flashing bare bum and bits of crotch as they walk around on stilettos that make them look like those fishermen's houses built on stilts – the ones that don't outlast storms or oil slicks. They all smoke cigarettes. Marlboros or Chesterfields with health warnings written in Cyrillic. And everything inside from the drink in your hand to the seat you lean on smells like candy floss.

As soon as I tell the women I'm just writing a story, they take out their phones, start eating pizza, and pluck stray hairs from their bikini lines—the same things they would have done if I'd told them I was gay, I guess.

The layout of the building is an homage to the penis itself; a long narrow bar leads into two tight networks of small rooms with wipe-clean beds and showers and lighting that you'd never want to read a book under. Europop plays from tiny speakers, hidden high up in the shadows. I've never been to a nightclub in Dubai but I imagine that beyond the mirrored dance floors, the Swarowski glasses and the $5,000 table reservations, the DJ's got the same bad collection as the King George.

"Do you like the music?" I ask Alina. "What music," she says?

The majority of the women are from Eastern Europe. Klaudia is from Austria and she's something of a celebrity in Berlin. Men request her for €200 (£158) an hour. Alina says she's from Napoli and that she misses the sea and her home. But both Alina, me, and her accent know her home isn't Italy. It's probably Romania. The same goes for the other girls speaking Spanish like Petronella and Barby. They learn it growing up in Romania watching Spanish telenovelas and they speak it because it's fun, she says. It's fun pretending to be Spanish, but in Germany where being Romanian is the only thing that makes people as mad as a murderer it's good sense, too.

Obviously they lie about their age. Someone who looks in their 40s is in their 30s and the 30-year-olds are all 19. But I guess that's just a symptom of the dishonest premise brothels are built on. The women act like the men are interesting and desirable, and the men convince themselves they actually are.

The men start arriving as the factories and shops close for the day. They're in uniform too: steel cap boots, Snickers work pants with some grey t-shirt tucked in so best to highlight the arch of their gut.

Sascha knows his clientele well. "We cater for taxi-drivers, the unemployed, guys who aren't making much more than €1,500 (£1,190) a month."

Sascha bought the King George over six years ago. It was a strip club before that. Sascha grew up in East Germany where his first taste of the oldest job in the world was renting out apartments by the hour for a family friend. When the Wall came down, Sascha moved south to Bavaria before deciding on a return to Berlin. "A smile is the most important thing in a prostitute," he says, "they don't have to be pretty, in fact it's often better if they're not. What you want is the sort of girl who can still turn on some charm after twelve hours sitting down with nothing to do."

Sascha comes across as likable. The girls support the argument. Klaudia tells me he's too kind. He loans the girls money. He bought her a €300 (£237) handbag. But you can't tell the other girls that.

But does he fuck them too? "No," Sascha says. "The minute you do that you're not respecting them as an employee. And it can cause problems between the girls."

But who does he fuck then? "I do have a girlfriend, but finding someone to build a family with me considering my business is hard."

The girls have families though. Klaudia has a 17-year-old. She picks her up from work at night and they go eat kebabs together. Klaudia is also a nurse. She's useful to have in the brothel. In the real world she only earns €1300 (£1030) a month. A good night at the King George gets her €600 (£475). She does OK as a prostitute. She holidays in Ibiza in summer, the Alps in winter.

"A lot of the money isn't even from sex. The men just want to talk or share a bottle of champagne with me," she says. "I'll often have three of them here at once sitting in the jacuzzi and laughing."

It isn't just about sex. Sascha's got it all worked out. The average flat-rate customer fucks 2.7 girls. The rest of that time, he's drinking at the bar, feeding coins into the poker machine, maybe even having a lie down in one of the rooms on his own.

"Traditional brothels," Sascha explains, "are uncomfortable for a lot of men. They rush you in and out and some guys get nervous and can't perform. Here, a customer can treat it like his own pub and they have time to talk to the girls."

The King George is open seven days a week but the girls are allowed to work a maximum of five days a week. "To regenerate," Sascha says, "Mentally and physically."

A woman might have sex twenty times in a night. I can't and won't ever begin to imagine how a person regenerates mentally from that.

The girls leave and come back. Katja from Hungary has two children and is a qualified care worker who can't find work right now, so she's back at the King George for the moment.

Does she like it? "Sometimes but not really. You're not supposed to like work, though," she says.

Sascha doesn't have a problem hiring. Some days there are even lines outside. "In other brothels a girl might not even make her cab fare," he says.

There are more red lights inside the King George than all the junctions in Germany. On a bad night, a girl will come away with €100 (£78). For every euro that a customer spends they make 50 cents. Extras – like blowjobs without condoms, anal, kissing – earn them extra money. Because it's Germany and prostitution is not illegal, they will pay tax on that, and their contribution will go to building schools, hospitals, bridges, boots for German soldiers in Afghanistan. Hydra, an organisation that fights for prostitutes' rights in Germany, estimates there are close to half a million sex workers in Germany. Two thirds of them are not German. Klaudia the Austrian is as close as it gets. She has a weathered tattoo on her shoulder. It was her first one as a teenager and it says "Love."

"It's silly," she says. Love? I ask. "No, just the tattoo."