Vince, we are on our third Prevost (a 1992, a 2002 and a 2008).
I have always figured costs along these lines.
Expect that you will spend 5 to 15% of the purchase price in the first six months to get everything lined out (fluid changes, tires, batteries, air bags, suspension and other issues). And yes on a $250,000 bus you may spend $25,000 to $35,000 to get everything up to speed. On the 2002, I purchased new tires, batteries, and redid the front suspension, replaced shocks and some airbags and fixed a few issues with the house and ended up spending $27,000 in the first year. On the first two buses I ended up in that range (5 to 15% range). On the current bus I am considerably less, but that is because I made sure the price included new tires and house batteries and the price of the bus was higher. Of course the year isn't up on the bus. So far I have spent about $7,500 on the current bus to service it and fix things that needed to be fixed (awning motor, air leaks, air bags, etc). This stresses the value of having a PDI performed by someone who knows what they are looking at so you can negotiate a price understanding what you may need to spend. It can save you a fortune.
Each year I figure $5,000 for general maintenance (oil changes and other issues that pop up). You may get away with less but on average that is what I have spent. That assumes that you use the bus fairly regularly. If you use it less often you may have a higher expense. Buses like to be driven and lived in.
I also figure new tires and batteries every five years (tires at $7,500 and batteries at $4,000 to $5,000 depending on the number you have). If I take the tire and battery number and accrue over 5 years that is $2,500 per year.
Add the tire and battery annual reserve to the $5,000 of general maintenance and you get a $7,500 per year number. Assuming 10,000 miles a year that is $0.75 per mile before fuel costs. Keep in mind that this doesn't address the inevitable OMG repairs that will need to be addressed. My OMG expenses included a blown 8V92 engine that cost $17,000 to rebuild, a $5,000 slide out repair bill and a generator replacement bill.
In the back of my mind with the 02 I always figured I had a $50,000 bill at some point for panel repair (it was an XLII), but I never did.
Buses are not cheap but they are a great way to travel. I find that I have less repairs with the bus than I did on a Fleetwood diesel pusher but the costs are higher. I haven't seen a hearse with a luggage rack and figure if I leave it to the kids it will only make them worthless so spending money on a bus seems like a reasonable idea.
Good luck with the hunt.