I paid $4500 for my school bus - and spent over $10K in materials / tools - If I was to sell, I would not worry about the labor put into it - since that was part of the fun of converting into a Skoolie - but I would likely sell for about $20K if I found someone willing to pay that. Heck - I'd sell it for $50K if someone was willing to pay that. I would build another Skoolie or Tiny Home.
Luckily, you can usually find converted or partially converted buses closer to what you are looking to pay. Some will sell for over $50K some for under $5K. Some are cheaply built and some are very well built. It's pretty much like houses.
Value is different to people. I don't judge people who will pay $50k for a converted bus - or $500K for a Phaeton RV. I just would rather spend a lot less - because I don't make nearly enough money to afford that.
That said - since Skoolies aren't factory produced or manufactured - value is all in the eye of the seller and the willing buyer.
That being said:
2000 International with 7.3L V8 Turbo diesel 210HP with Allison 2000 transmission. 330,000 miles.
Runs and drives like a bus.
$20,000 (dog not included).
(just kidding - I'm not selling my home. I'd be homeless).