It's all about weight, distance and amount of stuff (more stuff = larger, more expensive, aircraft).
Check out
Wrangell Mountain AirWrangle Mountain Air for prices in the McCarthy area (they offer sightseeing tours as well for those inclined to a saner trip).
Here is another multi day (and multi-river) trip for you to look at: Little Nelchina R. to Nelchina R., cross Tazlina Lake (~9 miles), then down the Tazlina R. Road access at both ends = no flying needed.
And, for a multi-sport trip, check out
here. (fun viewing for all on this site).
There are also van/bus shuttle services running the McCarthy Rd, Richardson Hwy, and Parks Hwy. Then, of course, there is always the old thumb (RV'er seldom pick up hitchhikers, but maybe someone here will give me a lift if I have a paddle or backpack with me....please).
I don't know what is appropriate, but in the past, I have tipped pilots between 10 and 20 percent. I suggest letting your appreciation being your guide. If you enjoyed the flight and they went out of their way to show you things, then share the love and give them a tip. Often air shuttle services offer more than just the air portion; they may be able to shuttle your vehicle, provide a safe place to park, and even a place to camp the night before your flight.