I have paid more than $20 for some specialist iPad apps, but you're right most people expect iPad apps to be either cheap or free, yet on a PC/Mac they're happy to pay (ok, begrudgingly accepting of the concept of paying) full price for the full-fat product. Why is that? Again, it's down to how most people see an iPad: simple content-consumption device they don't expect to compete on equal terms with their PC and Mac. Consequently, such apps don't get developed, despite the hardware being eminently capable.