Well, you can do something like this:
=if(A1>=100,(A1*2%),)
This will take the number in A1, check to see if it's 100 or greater, then multiply it by 2%. Of course, you can substitute another percent or even multiply a different cell in that row by the %; just change the second A1 to whatever other cell your discount calc is in. This will create a whole column (if you copy it down all your rows) with the discount amount, which you can then subtract from the original to get the discounted price.
Hope this helps
CND