I'm also a short adult (4'11"). I have never tried to find a spin cycle, but we are far enough out of the range of normal adult size that I'm not surprised that anything that would fit would be super-expensive.
I'm even having a hard time finding a good street bike. It isn't really the height of the bike, it's the length from seat/shoulder to handlebars that kills me. My arms just aren't long enough to avoid back strain, and I prefer an upright posture when I ride a bike. Also I'm older. I may not be 4'11" anymore, now that I think of it.
I know this doesn't really help, but maybe if you already have a decent street bike, using one of the stands to turn it into a spin bike might be your least expensive option. Or purchasing a street bike and a stand. Although the combination might be as expensive as the spin cycle, at least you can take the street bike outside in good weather, so it's sort of like getting two options for the price of one. Good luck!
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