How much does it cost to tailor / alter a suit?
It depends on what you have done as well as where. A dept. store's tailor will proabably cost less than going to a custom tailor.
There are about 50 different thing that tailors can do with a two-piece suit and it is usually like a menu with prices.
I found a great suit a few months ago that didn't fit right and making the pants a little smaller in the waist, cuffs, a little alteration to the jacket as well as few other minor changes cost me about $85.
If you have a $100 suit you don't really want to spend that much but if get a really nice suit you want it to fit exactly to your body.