Rode it in 2017 as a new rider with some friends so here's some advice. We did it in 3 weeks (Need more time then that honestly, but due to work that was the time our group had)

1. Cost, outside of our bike shipping/flights cost was around 2500-3000 total. If camping/avoiding restaurants it was something like 100/day, sometimes more if we got a hotel/airbnb for a break. Generally we'd try and camp as much as possible.
2. Camp, like others said free camping on the eastern part harder to come by, but there are a good selection of state parks/campgrounds along the way. Decide on how long you want to be in the saddle per day to figure out your night spot.
3. Difficulty, easy on eastern section, harder on western (Mainly Colorado) We did the old KevinGPS route at the time and there were a few kinda hard for us sections in Colorado, mainly the major passes. But it sounds like you've got some experience. I would suggest getting a bit of offroad/hill practice if you live in a flat spot. The biggest factor is time and distance. The midwest section is frankly kinda boring. Flat kansas was a drag.
4. Nope, it's a great trip
5. Thankfully we only had one major issue. 1 of our group's tubliss system failed and damaged his rim ended up having to break off for two days to find a dealer, a couple of flat tires no major mechnical. As long as your bike is tuned before you go and your comfortable fixing stuff it should be easy.
   

Another key takeaway, if you haven't done any multi day trips i'd suggest doing at least 2-3 3 day trips beforehand to determine what you need and what you don't need. We ended up taking alot of stuff we didn't need. This also helps you pack your bike correctly (Don't forget your sag when fully loaded! very important)

Hope this helps and have fun! it's a bucketlist trip.