you are just paying for your usage in advance. Once it's used up, you have to purchase more. Postpaid, you use minutes or data then pay after. So that's where big fees could come in if things are over your plan.
right now i'm on the $25/month, 250 minutes, unlimited text plan on my iphone. No data...but u can add that as a feature package if you want. Prepaid data is rather expensive. Also, the prepaid 3g is pretty slow also.
If you want to just buy data once to test it out you can do that. Once it's gone it's gone. It doesn't have to be on your account all the time like it is with postpaid accounts. So for example...if i'm going out of town and want data just for my trip i can quickly add it. Then once it's used up i don't have to add data again if i don't want to.