The first episode of Penny Arcade Adventures was the highest-grossing Xbox Live Arcade debut ever, earning $330,000 in its first three days, according to the stat-crunchers at VGChartz. The prior record-holder was Worms HD. In terms of units sold, On The Rain-Slick Precipice of Darkness was the third-highest performer of 2008 - even with its higher price point.
As VGChartz's Benjamin Schlichter says:
What does this mean for the future of XBLA? Penny Arcade proved that a high priced game can sell well, and doesn't require the gamebe $5, or $10 to get a high number of sales in the first week. It also shows that Episodic Content has a place on XBLA, as PAA is the first of a series. And finally, it also satisifed the XBLA Market's desire for another RPG. PAA is a hybrid turn based, and action RPG (from my understanding). So these factors, as well as it's name brand contributed to the strong sales.
Penny Arcade's Mike "Gabe" Krahulik had something to say about it on the pair's site:
Given that our price point was double the other games on that list I'm pretty f***ing happy. We fully expected some people to complain about the $20 price tag but we honestly felt like our game was worth it. Pricing something like this is tricky. Obviously you have to look at the length of time it will take someone to complete the game but you also need to factor in the quality of the experience.