Honestly, vet bills are, or can be, HUGE on horses. We've owned several, however are now out of it since our daughter is grown. My friend's horse just had a bought of colic, which is not that uncommon. The vet bill was in the thousands.
How do you know there is not an underlying medical issue, lameness, etc? Call your local equine vets and ask how much it costs for a pre-purchase vet exam. That will give you an idea what it costs in your area to get the vet to drop by.
Good luck with your decision.
:)