I just answered in your other thread. I think it was around $200 for the test and about another $200 for the serum.
The dermatologist will shave your cat's side and test individual spots with different allergens, looking for any kind of reaction. It seems to be more accurate than a blood test. We did both and received different results. The serum from the blood test did nothing, and the serum from the scratch test (skin test) brought relief within weeks.
My vet said $250. He also said that it was very inaccurate on cats with food allergies and was a waste of money if food allergy was suspected. He only recommended to use them when inhalant allergy was suspected.
We had allergy tests run on Jamie's blood a couple of years ago, to the tune of €800 ($1,076 at today's exchange rate, but actually under $800 then). I'm not sure whether it was worth it or not, as it basically served to confirm what we'd already suspected due to food trials. No skin tests were done (difficult patient), but the clinic felt because of his auto-immune issues that we should have his blood tested at a lab in France, hence the expense. We've been going by the results, though, and he hasn't had any more trouble.