Anywhere from $150 to $500 depending on your vet, the lab they use, what you want testing done for. We had 3 tests run on Sarah (this was about 10 years ago!) and one was food, the other was seasonal (pollens, trees, ect,) the last one was misc (molds, mice droppings, and if I remember correctly it was about $350 or so.
Once we found out what she was allergic to, they made a serum to make her no longer allergic to those things. That of course costed more. I had to give her shots of the serum daily, until it became weekly, then eventually monthly.
But in the end, what really cured her, was a high quality dog food that did not have any of the ingredients she was allergic to. Go figure!