Cowgirl, you've gotten a lot of good advice in these posts. Yes, the vet would be able to tell if it was dropped. He might not be able to tell if it had dropped and slid back up, vs never dropped, if it's not down when he checks. He could tell you what the low end of the surgery cost would be, but as far as the high end, that depends on how deep into the abdomen the hiding testicle is. I've read other posts on this forum where they've paid for surgery and still not been able to find and remove the crypt testicle.
As far as thinking about the costs, don't forget the cost of providing the post-op care. If you're a boarder, don't forget to include the cost of a private stall and/or run and the twice daily care he'll need for a while. He won't be able to be pasture boarded until those belly muscles are healed and he will need handwalking for a while after surgery because he needs to be able to walk to keep the swelling down and help circulation/healing. For an abdominal surgery, he will probably be restricted for several weeks. I would guess 2-3 weeks for severe restrictions and TLC for another month or two. (The vet could tell you that, too.)