Substrate can be relatively cheap if you mix it yourself rather than buy pre-mixed! If you can find topsoil (important that it doesn’t have fertilizer added in, and topsoil in the US is pretty cheap) and play sand. As your base mix it’s pretty economical. Like CMSReptiles said, add in some sphagnum moss, worm castings, charcoal and leaf litter (you’ll want a bunch of that on top of the soil too, for the cleanup crew to eat) and overall it’s not too expensive. We mixed up a HUGE batch for our 4x2x2 and a couple of other terrariums and I think it came out as less than half the cost of buying bioactive substrate straight.
You can definitely gather and process your own decor, and actually, your own leaf litter, too. For the leaf litter we boiled and baked. Boil for 10minutes then put a single layer on a baking sheet and bake on low at 200F, stirring often, for maybe 30. We baked because we wanted them dry for storage, but if you’re just tossing them in and on the substrate you can actually just use em wet. The isopods like that.
We foraged a lot of our own branches and rocks from the nearby woods, scrubbed and sanded them smooth and then baked them. (Be cautious baking rocks – they can explode. Big ones, you’re better off sanitizing in some other way.) The most expensive part of our big bioactive was actually the cork bark, because we used a fair bit as backdrop.
If you’re going to do a lot of DIY and gathering from the wild, I definitely suggest being VERY careful about cleaning, sanitizing and quarantine. Hitchhikers are sneaky.
Keep in mind you will want to process your plants to account for pests, too, especially if you buy from large plant stores. SerpaDesigns has a good video on that: you’ll want to do it ahead of time because after cleaning, you quarantine them for a while.
We also ended up getting beneficial nematodes in addition to springtails and isopods (I second powder blue/powder orange as a suggestion; porcellionides pruinosus is the proper name of the species and they come in white-spotted too!). The nematodes do a bang-up job preventing fungus gnats, which we had an issue with at first. I’ve also been considering predatory mites, because even with all our careful processing, wood mites got in, lol. They’re not problematic but they did make me jump when I saw them on my BP.
Another thing I wish I’d known when starting – when you first set up your bioactive, you’re going to get mold and you’re going to get mushrooms in the first couple months when it’s establishing. This will balance out with your cleanup crew eating them (my pods LOVE mushrooms) but while their population is still establishing, just do spot cleans when you see mold and mushrooms. Also be conscious of the air quality; if you’re sensitive to mold, you may want to run an air filter in your terrarium room while it’s balancing out. This is also part of why you want your bioactive to cycle for a couple of months before you add your animal.
Also don’t be heartbroken if you have some attrition with the plants. Expect to have things die on you unexpectedly, and be ready with some pre-quarantined replacements. Our bird’s nest fern did not thrive for whatever reason – on paper the parameters were correct – really a shame though, it was pretty.
As for plants to use, it depends on your humidity/heat parameters! For a ball python bioactive, we’ve had good luck with our golden and marble queen pothos (but NOT the satin pothos, for some godforsaken reason those all died), prayer plant, lemon-lime dracaena, snake plants, and fittonia (although the cleanup crew have been eating these ravenously lately and I’m not sure they’re long for this world).
Bioactive is a lot of fun and I’m glad you’re trying it out! My first terrarium has settled quite nicely, and it’s been running for about six months – I’m no expert, but I have recent newbie experience, hah!