well if you do it right, your exterior diameter of the tire will be the same. if you don't make sure to do this, your speedo will be really off, and you will have to get it calibrated again.
Here's my guesstimate:
18" Cobra R wheel- $200(stock)/$300(chrome)
Tires-$800
Springs-$200
If you want shocks and struts-$150
Labor for all that might be $500.
So in the end, you might look at $3000.