This is suggestions, so please consider the following:
Make a "cap" from which on all future mechs for a period are included. This cap does not have to be fix for the games lifetime - as mentioned i understand you need financal support and until a decent cap i am willing to do so. For example fix the amount to 500 or 1000 Dollars (think about Gamers,who already put in that amount of Money to ....still a game no matter how great it is). Once a player has supported you that well - he will get all new packages free. Every year, you take a look at that cap and if reasonable adjust it (up e.g. 100 Dollar if much Content has been developed - down if Player rates indicate a dropping Player base). That would be fair for both "sides" from my perspective.
Edited by skilly, 19 February 2016 - 08:05 AM.