If you have an extra guitar that's quite playable and you don't mind modifying it, it would be a cheap mod. You could groove the saddle and put two strings in the third string hole or drill a hole for an octave G slightly behind the other strig holes. Make a groove in the nut and put a banjo peg in the middle of the peg head.
I've seen Yamaha FG75s for around $125 that would work well here.