Usually if you check in online they'll send you a follow up email to let you know if your flight has wifi. I'm not sure if this is an exact science so perhaps you just have to wait until you board in the event of equipment change.
The cost is $5. This is the "trial" rate. I think I've used it twice. The first time was flawless but the second time the connection was sporadic at best and slow when it worked. I sent a polite email after the second flight, explaining that I realized it was a trial and for $5 it would be good value provided it worked but frankly it didn't at an acceptable level. They apolgized and refunded the charge.
Dan