What happens if you used 100 percent of your data plan for the iphone 5?

I used over 100 percent of my data for my iphone 5. I have the 2gb plan. How much will I be billed?

It depends on your contract. See the phone contract, terms and conditions.

Excess data maybe $0.25 / Mb… 1GB = 1000Mb.

If you are over by 1GB = $250.

Turn off the data until next billing period if you want to save money. See link below.

Depends from carrier to carrier. You didn't mentions yours. If you've crossed the limit, turn the data off and use Wi-Fi for internet, it's free.

When my data is all used up my provider puts me on 2g network. It costs nothing but so slow

That depends on the carrier you are with and the details of the contract you signed