One thing to consider also, as cold hearted as it may seem, if your snake DID test positive for IBD, then the vet can't do anything. If it does have IBD, it will deteriorate quickly and the oncoming end will be obvious. At that point you can have it euthanized and tested if you want to know for certain.
Myself, I would only test for something the vet can fix. In the case of IBD, it's not a long term illness in BPs. $400 seems incredibly high priced to get told "It showed nothing, but we can't say for sure" or "it's positive but that means we can just put your snake down."