A: This is where boundaries begin to blur. What is a dating site anyway? Ages ago, the main kind of dating site was the matchmaking type. You didn't do much online, and most of the "romantic activities" were confined to the bar, coffee shop, or living room.
So, let's break the answer up into two categories: the "offline/online site" that doesn't encourage (or even allow) users to socialize on the Web and the more familiar "purely online" dating site of the eHarmony, Match, and OKCupid variety.
The "Offline" Online Dating Site
Most expensive is It's Just Lunch. IJL doesn't tell you how much it costs, perhaps because it costs so damn much.
Reports from members and former members indicate that you pay about $1,200 to $1,500 per year. In case you can't work the math, that's $100 per month and up.
One forum poster rightly points out that the high fees are part of the selling point because they weed out deadbeats and con artists.
If you've ever cruised OKCupid (a free service), you'll notice many zombie accounts. These accounts have photos of unrealistically attractive women and little activity on them. These zombies are either con artists in the making or people who want to check out the scene without giving away themselves.
The Purely Online Dating Site
This would be eHarmony, currently costing $59.99 per month.
In fairness, eHarmony does have extensive backend operations. And they're more than just an "online bar." Whether good or bad, they do have sophisticated algorithms--continually updated--that strive to match up compatible people.