If you meet a few criteria, you can get an iPhone 4 from Radio Shack for as little as $25.
RadioShack is running a little special from Dec. 4-11 that offers a $50 discount on iPhone 4 or iPhone 3GS. You must buy new devices and not refurbs. You just have to be eligible for upgrade. This means you must be ready for a new two-year agreement.
So the pricing would be: $49.99 for 3GS, $149.99 for 16 gig iPhone 4 and $249.99 for a 32-gig iPhone 4.
The Shack also offers a trade-in program. iPhone 3G owners get $75 and 3GS get $125. So you can get a new iPhone 4 for as little as $24.99 if you want to sign up with AT&T for two years.