best to call the numbers mentioned earlier in this thread, as the cost varies from person to person. it is 10 bux a month until your contract is fulfilled so each month its cheaper.
for example, my upgrade is due june 1. i have the option of pushing the upgrade up 2 weeks (may 18) if i so choose, so in my case there is no buyout price. I still have one more upgrade since i was in the premier program before it went belly up hence why my upgrade is in june. i used my upgrade last june 2011 to get the 3D and now that year is here, to which i will use it to get the LTE for 200.00.
so call retention's/acct services and have them give you your contract buy out price.
OR, if you want to, go to sprint.com and go to the shop section to shot for phones. they will pop up a box and it will say if you have a discount coming or when your contract will be fulfilled (in days) as to when you are eligible for the new customer pricing.