background, LL minors softball coach here (10u basically, mostly 8s and 9s, most 10s are in majors). Our softball admin runs a TB team, and has a neice playing. local TB softball and baseball teams use our LL fields on sundays, and they pay for this, but pay lower fee if 80% (or something like that) of their players are registered in LL as well, so TB coaches really push to have girls play LL as well.
It is difficult but not impossible for travel and rec to coexist. But you need to be careful with the expectations from both sides. IMO, the bold above is part of your problem. It sounds like they get a reduction if the players are "registered" for LL. What you should do is make the lower fee contingent on those players actually "playing in 80% of the games".