If we're talking about heli-coils, the taps are not standard sizes. They are special because the inserts are coiled wire of a given dimensional cross-section, so the nominal OD of the tap doesn't correlate to anything standard. So when looking for a tap for heli-coils, you have to buy one that is "designed for" whatever insert you want. If you can extrapolate any information out of a heli-coil insert, it is that the the tap for it will be the same thread pitch as what you are trying to match, in your case 20 TPI. But you can't determine the OD.
So, yes, they are expensive. You need the tap, installation tool and the insert. I believe you also need a way to break off the nib that allows you to screw it in, so that means another tool.
You might just want to take it to a machine shop or simply tap to the next size up if there's enough meat to work with. To me, heli-coils aren't worth the trouble.
There's also a cheaper alternative. Look here: https://www.mcmaster.com/threaded-inserts/black-phosphate-steel-easy-to-install-thread-locking-inserts/system-of-measurement~inch/thread-size~7-16-20/
This solution does take a standard tap. And doesn't consume any more meat than a heli-coil. I prefer these over coiled wire any day because it's one piece. I probably should have just led off with this one.