For most of my cages, I use brown paper that I buy at HD in 140 foot rolls ($10 or so each). I haven't bothered to work out how the price per cage compares to aspen, but it doesn't seem expensive (well, maybe compared to free newspaper).

If you go this route, a sharp knife will save you a lot of time (razor blades dull too quickly).

I do use aspen in a few cages. If you have a feed store nearby it may be much cheaper than a pet shop...
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What goes up must come down...unless it exceeds escape velocity.