How exactly can you use an iPhone with Metro Pcs?

I have Metro Pcs and heard a while ago that you can use AT&T iPhones with Metro now. I've googled how to do it and there's just a ton of different answers and i'm confused.
I want to get an iPhone 4s. I know if you buy one with a plan (att, sprint, etc) they're usually $99. But can you just buy one, without a plan? Would it cost more? And how do you get it unlocked to use with Metro?
Please help me out!

Added (1). Also the Metro Pcs website says you would have to buy a sim card to go with the phone. How much do those cost?

Okay I just getting an iPhone for Metro Pcs. And if you want an iPhone I would just buy one online honestly, without a plan. Now to get it unlocked you need to take the phone to a repair shop, or a phone pawn shop(where you could also find an iphone, I know gamestop as well sells iPhones.) Just browser the internet and websites for an iPhone without a plan. Unlocking the phone at a phone shop usually costs around $30-40.To get the phone with Metro PCS service will cost $25. $15 to transfer service to phone, and $10 for the sim card THEY will give you. If you have an unlocked iPhone. You can walk into any metro store and get it activated and put on their service. Overall the average used iPhone 4s sells for around $175 plus to unlock if hasn't been can cost another $30-40 and a definite $25 dollars when walking into Metro PCS. So you are looking at what I would say at MINUMIN $200 cost to get an iPhone activated to Metro PCS. Hope I could help, hope you find the best costs out there goodluck!