Open the 0.mi file from the "F3" mode in the GEM editor, press "6" to open the triggers list. Now click the "difficulty" trigger and find the line "if hard", then change the value of "reinforcement" to C+=1500 (or the amount of cash you want to start with). It might be capped at 2000 by default but that can be changed as well.
I found it a little trickier to make the map appear in game as it was modified. You should try to load it as a mod, which means you might need to learn how to make a new mod containing all your modified maps as well. I suggest you check out the "skirmish variety mod" for an example of which files you need to modify the original skirmish missions.
http://www.moddb.com/games/men-of-wa...sh-variety-mod