I've always thought that $US 20 is an awful lot to spend on a casual game. One of the reasons the conversion rates from trials to purchases are so low (around 2%) is that there's often enough pleasure in the trial to satisfy.
Advertising to the rescue. MostFun.com is offering free online versions of popular casual games like Diner Dash 2, Chocolatier, and Luxor 2 with no time limits. All you have to do is watch a pre- and post-play ad. Is it worth it? You'll have to be the judge; their player doesn't work yet on my Mac (or yours either).
Free Unlimited Premium Games [MostFun.com]