Good post... Nice read to let new-comers know what the reality is.
As far as "mobile repeaters" go... to do it right, its much more expensive and difficult than a fixed-station repeater, with nowhere near the performance advantage. I built a portable repeater system for a government emergency response team. It cost a bit over $16,500.
The antenna alone was a massive project. It was on a 10' tall tripod (mast mount centered half way up) with a 30' fixed mast and a 30' crank extension, for a total of 65' of elevation. The equipment and batteries were extremely heavy and had to be installed in vented enclosures to keep them dry and cool. The repeater had to be trucked to a location and dropped at a high-point. Setting up the portable tower took a huge amount of labor and real estate to get it stood up and staked out so it was straight and wouldn't fall over in 20-30 mph winds.
And unless you have a babysitter, you need self-contained GPS tracking and local alarm on the transceiver in the event that some knucklehead finds your repeater and decides to leave with it.
That all depends. I have a Ham buddy who built a special event repeater, for local marathons and bike runs, really cheap with some used radios and a good power amp. The way it was done is using cross-band repeating so the expensive cavity filters get ditched. Most Ham HT’s can be programmed to do split band operation. You RX on one frequency on one band, say VHF, and TX on another, UHF, when you press the PTT button. For Ham radio this usually ends up being the 2 meter and 70 cm bands. On the repeater side a simple diplexer is enough to work the VHF and UHF radios into and can be had for $50 to $75 new.
Then he takes advantage of the local high rise buildings. Through his contacts he has access to the roof tops. No need for an expensive tower and feed line. When you’re on top of a 10 to 15+ story building you get some decent coverage. Since the repeater can be located close to the temporary antenna you don’t need expensive feed line. And what you do use is fairly short thus keeping the losses down and thus the TX’er power required.
Granted his conditions are not the general case but if one looks around and takes advantage of the local resources you can save a huge amount of money and effort.
However if one wants to build an in-band repeater, then yes it gets more complicated and likely expensive as you pointed out.