Some years ago, I think I paid possibly in the region of £50 per bolt for a slightly smaller boat. They were an odd diamater and custom made from threaded rod.
Do not fo to a stainless manifacturer to save money, make sure you get them done at a marine stainless steel fabricator as keel bolts are from a different grade of stainless steel as to most stainless steel products.
I had mine done about 10 years ago on mirage 30 (bilge keeler) the 15 years old in order to satisfy the surveyor. When the yard removed my keel bolts they were found to be bright with no corrosion at all - they were mild steel.
It might be a good idea for you take some time to review the process of anerobic corrosion of stainless steel and then decide if mild steel is not the best option as a keel bolt material.
Sorry but I cannot remember how much this lot cost but it was hundreds, over 500 and under 800 from memory.