No, I am not trying something different with groupomatic. You hit on the head. I am trying to be a better meetup.com. I imagine it will be a slow, long haul, but it doesn't hurt to be free :). I tried to start out as simple as possible and not just copy all their functionality. In so doing I've already made several departures, errr improvements imo, e.g. group scheduling, no forced site registration, group specific handles, etc.
http://www.amazon.com/Strategy-Tactics-Pricing-Growing-Profi...
It was on the reading list for a pricing class I took. All I can say is that pricing should be the culmination of your marketing strategy. The book does a great job of explaining some real rationale and logical thinking on how to set your price.
The nice advantage you appear to have is that you can tweak or iterate your pricing strategy? maybe try different prices for different people or times of the day/month/year?
Good luck!
My last startup charged for a web-based service. It was a consumer impulse buy and so we tried to keep the price down for that reason. When we got enough size, we ran statistical tests to figure out the best price point in terms of maximization of profits.
I also have had experience selling an online book. As you suggest, sometimes the demand curve is not a straight line and raising the price can increase volume because prices can of course influence perceived value. I raised it from $9.99 to $24.99 and that is exactly what happened--increased volume.