How much is it to upgrade to an iPhone 4s?

I'm with Virgin Mobile and I currently have a blackberry curve, I'm pretty sure that the contract is up on my phone now so I was considering upgrading to an iPhone soon. The plan on my blackberry is $35 monthly for unlimited texting and bbm and calls after 5 o'clock and on weekends.

Do you know how much it would be to upgrade to an iPhone? Also I would like to keep the monthly plan under $55 if that is possible. And can you please tell me what some good iPhone plans are?

Most places it costs about $100 to upgrade to a 4S. I upgraded to a 5 for $200 with a contract and its nice. The iPhone is definitely worth it, and my monthly bill is $30 for unlimited talk/text and because I wanted the extra data. I'm on AT&T though.

You can maybe even get it free with a really expensive plan