Would it be a spoiler to tell us why you are working on this?
Is it to allow perma MP3 and MP4 or all the alts so they can help out new players?
Is it on an exeption base, special case and not really intended to use often?
Depending on the answers to the questions the answer to your question could change.
If you want to keep forcing people to advance MP at least at some point and afaik this is still the case, as in they should or it is intended, then you could also scale the cost up for each use, this way newer players can get their heat reduced if they want/need to, but you also restrict perma MP3/4 by making it harder each time they use it. (assuming reset to 0 heat)
If you want to allow perma MP3/4 then a fixed cost would be best, coins would be good since you can take them out of the game that way