I remember that thread..... but I can sum it up in (hopefully) a few well-thought-out sentences....
It can cost as much or as little as you want it to. Count on spending at least $2500-4000 for a decent shell to build in. Where you go after that is entirely up to you.
Let me preface the following by saying all of this is strictly my opinion. Others' opinions can, and will, be different. I'm not saying my way is the right way, or the only way, but this is the way I will be doing mine. Your way may be different, which is not to say it's wrong. If it works for you, it's right. That being said....
You can drop a couple of thousand on some furniture and a couple of cabinets and do a very minimalist conversion (which some people on here have done... throw it in, fasten it down and go), or you can drop 50 grand and end up with something to rival the quarter million dollar S&S rigs you can buy. (S&S = Sticks and Staples, which is how the commercially produced rigs are built. Dirty Words around here.)
Within each category the cost can vary widely based on whether you buy new or can find decent used items such as cabinetry, appliances, etc. Some things you will absolutely want to go new (like anything electric-related, by this I mean breaker panel, wiring, outlets & switches, etc). Likewise with propane and water you'll probably want new tanks, plumbing, etc. The fresh water tank you absolutely want new. Anything that can hurt you or kill you, don't skimp on it to save a few bucks. Get the best quality you can.
Absolutely STAY AWAY from RV type refrigerators unless they're helium based. Google "RV refrigerator fires" and you'll understand why.
Other things don't matter as much, and you can source from auction sites, Craigslist, etc. Habitat for Humanity REstore (is that the right name) is a good source for gently used cabinetry and such... looks good and is cheaper than new. If you're handy with tools and have the room you can fabricate a lot of that kind of stuff yourself and save money as well. If you don't have room, try to find a makerspace near you and for a nominal charge you can use their shop to build. This varies by location though, so do your research.
Best advice I can give.... read everything you can here before you pick up so much as a screwdriver. It can save you some costly mistakes and show you some valuable shortcuts to save time and money as you build it out. And if you have questions, ask away, there's a ton of people here knowledgeable in just about every discipline needed to do a conversion (carpentry, plumbing, electric including green power sources, and HVAC are the big ones; also people knowledgeable in gas and diesel engines and drive trains).
We're also a very visual group so we will expect pictures and/or videos of your conversion as it progresses.
Now, to answer your question... if it were me doing what you're proposing.... I would probably spend $10-15k (not including bus) to build something that would suit. For my own bus I've probably spent 6k and I'm still lacking some critical items... water tanks, electrical wiring and fixtures (though I do have the panel and items needed to make shore or generator connection), and battery bank are the main things, which will probably add another 3k to that total.
(Ok so I promised a few sentences and delivered a book. I do get a little long winded sometimes.)