I think you have 15 days to dislike a NEW phone on Sprint and switch to another (for a fee, I think). If you're holding a USED Evo (older than a couple weeks) you're stuck unless it's time for an upgrade. Sprint used to offer money for used phones when upgrading; I don't know if they still do. If it's not upgrade time, you're on your own.
Possible solutions become eBay and Craigslist. You can sell and buy used gear on those sites. A new iPhone 4 will cost you full price from Apple which will be hundreds of dollars.
We do offer a program where we'll buy back your used phone --
You can look there to see what the value of your Evo is.
Juann, out of curiosity I did the research. You CANNOT buy an unlocked phone from Apple for CDMA networks (according to Apple). You can only buy an unlocked phone for the iPhone 4 for GSM networks; i.e., AT&T. In November, Apple will begin selling an unlocked version of the 4S, but it also only for GSM networks.
I've included links from Apple about this below, including the prices for the iPhone 4 and 4S on contract AND the unlocked prices. Check out the fine print and the prices:
As far as Sprint prices on contract, those prices are on this site at "Shop", "Phones" and "Shop", "Plans" from the menu bar at the top of this page. So for example if you buy an iPhone 4 for $99 on contract from Sprint, you will also be obligated to a two-year smartphone data plan. You'll probably need some unlimited plan with the additional $10 smartphone fee to get what you want.
You could possibly find a USED iPhone 4 for Sprint's CDMA network on Craigslist, but this phone is essentially brand new, so it may be hard to find and will likely sell at a full unlocked price.