It would run you about 350 pesos at any of these places, the experience is same as you would have in the US or any where else (Show ID, fill a form out, etc.)Yes you can still get your CDMA connected at Claro or Centennial.
I just unlocked a locked/used US phone (without sim) and obtained 300 pesos of Claro for 300 pesos in SPM.
For the Orange SIM cards I can usually get the phone unlocked (with a new number and card put in) for 200. Depending on the phone, it can run 300. They almost always start out at double or triple that amount. I use a little independent shack that is around the corner from Iberia in SPM. Over the past 2 years I've probably had or taken people to have 8 phones unlocked and new cards put in so they usually don't hassle me too much anymore with the price. However, my first real argument in Spanish was at that very store when they tried to rip me off (I had just unlocked an identical phone the week before and the guy insisted that I couldn't have been charged 200 pesos--until the very pregnant assistant verified that she and I had a long discussion about babies and breastfeeding!)