Because Monopoly money doesn’t have a WWII theme printed on them… kinda the same thing when I ask why don’t you just use some of those little square armies from Risk for your infantry instead of molded soldiers… I’m sure it would be cheaper. Sometimes, when making up a custom set, you’d like a certain “look”.
$7 isn’t that bad… i’ll make a sandwich today for lunch instead of getting a happy meal… there… now I can afford Battle Bucks.
If I wanted to save dough that badly, I’d just use a few thousand Canadian dollars as game money… I think that’s worth about one or two American dollars.