"Crushed" (broken) Ceramic tiles will work reasonably well. Just avoid the ones with bright colours (as they sometimes contain various metals that may leak into the water) - white ones are most likely Titanium Dioxide, whcih is completely inert [Titanium as a metal is expensive not because it's rare, but because it's TERRIBLY difficult to split up TiO into Ti and O]. You can probably go to your local tile center and ask if they have any boxes that have been dropped or damaged, and get them for pennies [no, I haven't tried that, I'm just looking at what the regular filter media actually is].
What you really want is anything with a large surface area and lots of little crevices. I didn't think BioBalls were that expensive, but I haven't ever looked at the prices (my Eheim filter came with what I think is good substrate...).
You may also be able to use Leca (Lightweight Expanded Clay), which is used for pot-plants and such - and also for making light-weight building blocks. So, in big sacks at the building suppliers or some such, it should be relativley inexpensive.
Having said that, it's a one-time purchase (or at least it should need replacing VERY infrequently), so I'd rather buy the stuff that are produced for the purpose and know that it's something that works and doesn't have any problem side effects.