Ex mechanic and DIY rider, here:

Yeah, $300 is a wee bit high. For that kind of money you can practically buy a a new pre built hub motor and wheel IF you can find something that's compatible without a huge time sink chasing down returns, etc.

It's about $150 around here for a normal wheel rebuild using standard sized spokes, including parts and labor at around $1 minute. That may be a friend of the shop price, but that's about right for a full spoke replacement and truing

With a hub motor wheel specs like spoke length are often all over the place. so that means cutting and threading custom spokes or hours on the phone and email trying to get replacements. In either case time and labor costs go up in a hurry and a hub motor rebuild is probably about twice the work and hassle of a normal wheel.

There's also issues with working on some ebikes because their frame designs can be all over the place in a bad way. The more that your ebike looks and works like a normal analog bike the better. The more your ebike resembles a fat tire scooter or moped, the more likely it's going to he a pain in the ass to work on and take more time/costs in labor and chasing down non standardized parts.

At my local shop, for example, I once saw them do a tube replacement on a fat tire moped style ebike and run up a $450 labor bill because they had to take apart the whole back end and frame of the bike just to get the damn wheel off to get at the tube and tire.

And if you looked at the frame and bike it looked like the wheel would come off like a normal bike, but nope. Brake caliper had to come off first, and for the caliper to come off you had to take apart the two piece rear frame first. The calipers, then wiring harness, and then finally you could get the whole wheel off.

And then they couldn't get the thin, floppy and really cheap OEM fat tire tomseat on the rim properly even when using tubeless tire mounting skills and high oressure air. Every time they got the bead to catch and lock it was out of round and hopping. These are all issues caused by cheap rubber and rim channels.

Anyway, a new tube and labor is usually like $15 and maaaaybe a 10 minute job. That one was like 4 hours and $450 of labor on a bike that was probably under $1k brand new.

So... yeah. $300 is a bit high but it's in the ballpark.