Does anything in RAGE’s multiplayer mode unlock anything in the single-player campaign

After playing through, the best answer I can come up with is that there really isn't a recipe good for your personal economy. Instead, craft lots of wingsticks and pop rockets and go do a few rounds of Mutant Bash TV. You can average something like $600-$700 per episode.

You can trade 20 of them to Jacob, he's in the bar in Wellspring, for a defib upgrade (which means you can die one more time before a game over)

There is virtually unlimited Feltrite in the game so selling a few doesn't matter. driving around the wasteland will occasionally activate missions automatically. One of the missions requires you to collect 7 falling Feltrites, afterwhich you keep all 7. I just recently came across another one where it was "collect as many as you can in the time given" - I got a whopping 15.

Note that the Feltrite shower events (Feltrite tempests/storms) are not unlimited, but scripted. There are 3. The first two will allow you to get up to 7 each. The third one will allow you to gain up to 20.

As far as any other use outside that one upgrade, I don't know of any.