These suckers are the reason a $30 hot dog exists.

Dopey tourists are paying top dollar to bogus vendors for “tickets” to the Staten Island Ferry — which has been free for 18 years, The Post has learned.

One clueless pair shelled out $400 for a round-trip journey to the city’s southernmost borough, sources said.

“They’re targeting people who are tourists, because they just don’t know better,” said a source with the city’s Parks Enforcement Patrol.

Career con Gregory Reddick, 54, hooked the big fish on Wednesday while wearing a snappy “Authorized Ticket Agent” jacket near the South Street Seaport, officials said.

“Usually, the people we’re seeing are complaining that they’ve been charged $25” for a Staten Island Ferry ride, said the parks-enforcement source.

“But this guy was charging $200 each way.”

Reddick, of Jamaica, Queens, was caught by Parks Enforcement Patrolman Jean-Baptist Joseph, 33, on Pier 15 as he allegedly took the cash from the two marks.

When Joseph asked to see an ID and Parks permit, Reddick shoved him in the chest and took off, a criminal complaint says.

Joseph chased Reddick to The Battery, where he and a half-dozen fellow officers surrounded the 200-pound suspect, subduing him with the help of pepper spray, the complaint says.

He allegedly still had the $400 on him, but the two tourists were already gone and the cash could not be returned, a source said.

Reddick’s rap sheet dates to his early teens and includes six felony convictions, other law-enforcement sources said.

He has at least five aliases, six Social Security numbers and seven dates of birth — and has spent at least nine years in prison for burglary and credit-card fraud, sources added.

“We’ve been getting a lot of complaints, but nothing like this guy,” said one officer with the Parks Enforcement Patrol.

City parks enforcement patrol officers have seen an explosion in Manhattan tourism scams recently, including sightseers conned into buying forged, expired or overpriced tour-bus and boat tickets.

At least one sucker was tricked into paying $50 to get into The Battery, which does not charge admission.

Joseph Puleo, president of the union local that represents the city’s 240 Parks Enforcement Patrol officers, called Reddick and his ilk “thugs” who threaten his officers and tourists.

“This scheme with the Staten Island Ferry is something that recently exploded,” he said.

Reddick would not comment.

Reddick is one of a small army of unlicensed tourism “ticket agents” who wear vests reading “SJQ Sightseeing” — a company that has no Parks Department permit and is not registered to do business in New York state.

PEP officials and legit vendors say that, at best, SJQ buys valid bus and boat tickets in bulk, and sells them at a profit.

At worst, they sell bogus tickets that aren’t worth the paper they’re written on.

“They sell our tickets for whatever price they want and they tell people all kinds of lies, like, ‘Oh yeah, there’s free parking!’ and ‘Oh they’ll give you free food and beer’ and all kinds of lies,” complained one ticket booth agent for Hornblower Cruises and Events, a legit outfit.

Reddick, meanwhile, has been released without bail on the slimmest of misdemeanor charges, despite his history of multiple warrants and parole violations — and prosecutors request that bail be set at $1,000.

With no “ferry ticket” victims in hand, and without injury claimed on the part of the parks officer, he was only charged with resisting arrest and disorderly conduct.

“That’s a slap on the wrist, and a disgrace to the people who protect our parks,” Puleo complained.

Additional reporting by Antonio Antenucci, Rebecca Rosenberg, Melkorka Licea and Laura Italiano