I need to mod the Yamato Cannon to cost minerals. I'm trying to make a sort of 'ammo' system, where the mineral counter acts as the amount of times the player may use the ability. This would allow me to place ammo caches here and there. Any ideas?

Plugin, natch. Do you know C++ and have access to GPTP? If so, do a unit loop and check when unit->mainOrderId is whatever Yamato's order is, which can be looked up in enumerations.h or order.dat. Inside that block, check if the player's minerals are high enough; deduct if they are, and otherwise set unit->mainOrderId to 1 (which is Stop).

No.

I've asked about this before; see here.

Won't that cause the minerals to be deducted for every frame that the order is being executed, instead of just once?

Yah, that's what I ran into and why this isn't done yet. >.>

Thanks for the service, nonetheless. This is well beyond me.