Low budgets are never the problem with Christian movies. Low budgets are never the problem with bad movies, full stop. What's the old saying? It's a bad carpenter who blames his tools? Most viewers (and certainly most critics) are discerning enough to make allowances for the limitations of technology. "Well, we did our best with a low budget" is an excuse that Christian filmmakers have used for a long time to excuse what is actually shoddy craftsmanship, and it's disdainful of the audience, to boot.That doesn't mean it's not an economics issue, though.
Typically, the biggest problem in Christian films is something that doesn't require money at all: the writing. (Of course the screenwriter should get paid, but it's not like buying a better camera.) Christian films rarely tell stories with anything like nuance ... The single best thing Christians can do as filmmakers is to spend more time on their stories, to workshop them, to develop and hone the craft of writing.
A lot of times, nuance doesn't sell. But there's a big market for bad art, whatever its religious commitments. The question of why particular art gets made is not unrelated to the question of how particular artists get paid.
There's a scene in "Andre Rublev," the 1966 Andrei Tarkovsky film, where the icon painter complains he wants to paint a picture of redemption but can only get patrons to finance the apocalypse. That might still be true.