I've been looking for low tuned A and C for less than $50 to no avail. Are they rare in blues? Do they overlapin scale too much with other harps?

Are there low tuned harps in Ab, Bb, Db?

Seydel do a very comprehensive range of low keys - right down to Low Low F.
Hohner go down to low D in a few of their standard models, low C in the 14 hole Marine Band and they have the low tuned Thunderbirds.

Low keys aren't hard to find - it's the under $50 bit that'll make it trickier.