A helicoil tap is larger than the original 12-24 to allow the helicoil to go in. Helicoil taps are a proprietary size for that purpose, not something you can get anywhere else. And 12-24 is an odd size, the reason for the high cost. I have one I bought years ago to repair a Holley and spare coils, if you are near Orlando I would be happy to do it for you. You can check prices from BLP, www.blp.com
, or ask them the cost to send your body to have it done. If you have any other major carb shop near you , they could do the same as well. One last possibility, see if a local NAPA can order it cheaper.