The Thetford porta potti I have in my DIY campervan was $250. Chemicals cost about $10/mo. It has a 5 gallon capacity which lasts just a couple days with myself and my partner using it (I built a separate bathroom in my van) but we're often out in the woods where we do a lot of our business outside to make the tank last longer. Emptying usually just requires a quick stop at a vault toilet that are common at the mountain bike trailheads we frequent.
Thanks!
Around $1000. Cassette toilets need more space, and they are permanently mounted, but they are proper flush toilets, unlike porta potties, paint buckets and other such chamber pots. The cassette is larger too, so you can use it longer.