Should be pretty easy to figure it out. The Evo will cost $79.99 a month. Minus what you are paying now and it will give you the difference
That's for a single plan.
If you are on a shared plan, which it has to be everything data, it will only add $10 a month.
If you are on one the the older family plans that has added internet and all the other goodies like the old days, you will need to either switch your phone to a standalone plan or change your shared paln to the everything data plan.
So it all depends on which plan you actually have and what you will need to switch to.