Hi,
I actually have a boxer with Cushing's. I can tell you that we had an ACTH test done it was around $250, but that varies greatly from vet to vet. There are 2 tests one is the ACTH and the other is the LDDS. If the test does not confirm either Pituitary or Adrenal cushings an ultrasound is usually performed, but not always. Depending on the medication your vet prefers (we use lysodren) the cost will vary. The testing is the costly part the rest is not bad.
I will tell you from my personal experience if your vet isn't up to date on testing protocol, or hasn't had much experience with Cushing's - look for another vet that is experienced. Dexter had the Cushing's symptoms, drinking a lot, urinating a lot, accidents in the house, panting, eating all the time, lethargy, and hind quarter weakness. He was diagnosed in Dec. and our first vet was terrible, couldn't get him regulated on lysodren because he didn't do the proper testing. We are now at a different vet that does the proper testing and works closely with an internal medicine vet - he is doing much better. His drinking and eating is back to normal, he goes on walks again, and likes to go outside again.
I hope this has helped. If you would like more information an excellent forum that is much like this one is caninecushings.net
Christy
Dexter & Sophie both 9 1/2