Is $150 a fair price just for labor to swap an interior door?

$150 is ridiculously cheap. Most pros and decent handymen won't even back out of their driveway for under $250 or $300 these days, so your brother-in-law's buddy is basically giving you a mate's rate. As for trying to do this yourself with a hammer and a cheap screwdriver? Don't do it. Hanging doors looks simple from the outside, but it's one of the most frustrating trim carpentry jobs you can attempt without experiance. If the rough opening is slightly out of plumb or the frame has settled—which is true in almost every house—getting the gaps even and the latch to actually catch without rubbing will drive you insane. A lot also depends on whether you're buying a **pre-hung door** or just a blank **slab door**. If you buy a slab to fit into the old frame, you have to mortise out the hinge pockets with chisels, drill the lockset with hole saws, and often plane down an edge to get it to clear. You don't have the tools for any of that. Buying decent chisels, bits, and a planer would eat up half that $150 budget right off the bat. Even with a pre-hung door where the jamb is already built, you still need a four-foot level, shims, a pry bar, and finish nails to set it right. Hand over the cash and save your weekend. Just ask him first whether he expects you to buy a pre-hung unit or a slab so you grab the right thing from the lumber yard.

Take that deal before he changes his mind, seriously. Just make sure you check if that covers putting the trim back up and hauling off the old junk door.

If you're going pre-hung, the casing's gotta come off the wall first. Some guys'll just leave the trim off or ask for extra cash to nail it back on. Even if you've gotta do the painting yourself, you're getting off super easy. I'd jump on it.

Pay the $150, but hang out in the room and watch him do it. That’s what I did on my first replacement. I stayed out of his way and just watched how he set his cedar shims and swapped a short hinge screw for a 3-inch screw right into the framing stud to pull the top gap tight. Seeing a guy do that in real time taught me ten times more than YouTube. Cost me the labor once, but I was able to handle the other three bedroom doors down my hallway later by myself.