Bioactive for beardies is very tricky and dangerous, it has to be done very correctly for the beardie to live a proper healthy life without any problems. And it does cost a lot to do.
Vet bills are pretty expensive so the cost of substrate is probably the least of what you should be worrying about.
Personally I'd say minimum for a tank thats bioactive would be 6x2x3 at the very least, 6 being the length, 2 being width and 3 being height however I've seen one person do a fairly good bioactive enclosure thats 4x2x4 I believe with tons of climbing space, tons of basking options and tons of options for UVB levels.
You want to have tons of climbing space, this is very important even when not doing bioactive as they are semi aboral lizards however if you dont offer enough climbing space in a tank thats bioactive they will get fungal infections, scale rot, joint issues, basically all of this bad stuff that is caused by being on a damp substrate and not being able to get out of it.
This isn't mine but its the enclosure that I was talking about thats 4x2x4.
Personally if I did bioactive Id do a 6x3x4 or a 6x4x4. It would have tons of climbing space, shelves, rocky outcrops, branches, logs with different temperatures and UVB levels and then for substrate there would at least be 3 feet of hard rocky/clay on one side with hides and basking areas and then about 3 feet of either biodudes saharah mix or my own mix on the other side with a low wattage heat lamp, and LED lights just to light/warm the area up slightly to about 80 or so and also provide light for the plants if I did put any. Most likely a aloe vera plant, maybe some good cacti and elephant feed.
