Capcom's Dead Rising Case Zero sells over 300,000 copies since its release last week. The "demo's" price of $5 turned out to be very popular and now it is rumored other companies might start to do games that seem like demos but cost money (ex. Acitivision, EA).
The last paid "demo" that ended up successful was GT 5 Prologue which helped fund the last couple of years of development for the main game.
http://kotaku.com/5632494/dead-risin...el-sells-a-ton