There's a new mod for Portal 2 called Aperture Tag: The Paint Gun Testing Initiative, which replaces the game's traditional spacetime-bending portal guns with a different device entirely, one that shoots special gels that enable you to solve puzzles by bouncing or running at superhuman speeds.

It features a campaign mode with two or three hours of gameplay, as well as voice acting and an original score. It also comes with a $7 price tag.

Aperture Tag is the first, and currently only, game mod on Steam that isn't available for free. Despite the fact that it passed through the Steam Greenlight voting process to earn that spot, the Steam community is in an uproar that the mod "actually has the gall to CHARGE MONEY," as the top-rated review puts it. There are some positive reviews as well, but they're buried under pages and pages of negativity.

Why are people so pissed? In the specific case of Aperture Tag, the general consensus is that it's simply not worth the money. But the reaction raises a bigger question: Would gamers ever find it acceptable to pay for a mod?

There's always been something of a tacit understanding that fan-made modifications to commercial games are supposed to be free. But is there really anything wrong with modders asking for a small payment for their effort? Modding, content-creating, whatever you want to call it—it's all a form of game development.

On the other hand, mods are built upon existing games. Though presumably featuring some sort of original content, they take advantage of assets, characters, mechanics and more that were built by the game's original developers. Even the notion of the paint gels isn't original—it was included in the latter half of the Portal 2 campaign.

I see it as coming down to an issue of quality: Most of Aperture Tag's complainants cite its poorly written script, sub-par voice acting, and general lack of polish as reasons why it is not worth $7. Many say they'd be perfectly happy with the mod if it were free. $7 on Steam goes a long way, since the digital distribution service often features huge discounts on retail games.

Many people, myself included, had no problem throwing down a few bucks for a game like The Stanley Parable, which originally began as a free mod built in Half-Life 2's Source engine. But in that case, the original mod was free, and the creator only charged for it after it became a standalone game experience.