I bought my Photon off of Craigslist new and activated it on Platinumtel for $40.
The phone needs to have clean ESN and not be on Sprint account.
The later is the hard part, the ESN can be clean but the phone can still be on an account and not be active.
I went through 3 phones from Cowboom to find the last part out, which is why I found a new never activated one.
If the phone is on your account and you still have an account with Sprint you have to call to make sure they take the phone off of Sprint.
I pay about $3.33 a month for PlatinumTel and use Google voice and GrooveIP on wifi.