A lot of horses around here get leased (during season) anywhere from 3k -30k/month! So, the price seems quite fair especially for a packer.
Obviously don't know the horse, but the lameness makes things more tricky... Especially, like someone else mentioned, in contractual terms. How does the contract define the lameness? If the horse has passed a vet check like the owner stated, ASK FOR THE RADIOGRAPHS and have an unaffiliated vet take a look.
It might be the attorney in me nit-picking, but if they refuse to release the vet findings, I would think there might be something hidden.
Why not have the horse vetted?