Cheapest substrate is non-scented/dyed cat litter (100% clay) and some peet under a layer of gravel. People have also used some brand named topsoil (without NPK) with some success.
Next would be the mineralized soil since it takes a few weeks/month to get right...and good weather.
If you don't mind paying for substrate go with Flourite from Seachem, Eco-complete, or that ADA soil (several varieties).