I own several nondescript watches from the '20s - '50s all of which cost (or were gifts that cost) $100-$300. (The watch in my avatar is one.)
For these, I've found that they need serving or some repair and servicing about every five years and that costs today (used to be less, but I don't own a time machine) $200 - $300.
Effectively, maintaining these old watches means you will all but "rebuy" them every five years and will pay several times the cost of owning them over the years.
To several of the comments above, I agree, there are no shortcuts - you need to find a good repair person and they all charge about the same. Sadly, in the twenty-plus years I've owned old watches, many of the old-time repair men have passed away and I'm finding it hard to find good, reputable ones now.
I have a few friends who own Rolex's, Tag's, etc. and they tell me that the authorized dealers for these watches charge three, four or more times what an regular watch repair guy will charge, but if you don't go to the dealer, you void the warranty, can hurt the resale value of the watch and, if the guy isn't good, end up paying more to have his bad work undone.
Some of those friends only go to authorized dealers and some have a good repair guy they trust and they use him to save money once the watch is out of warranty.
Several years ago, I bought a circa '50s Girard-Perregaux for about $250 that needed a new crystal and servicing. Stupid me, I went into the local GP dealer and was quoted (from memory) about $800 ("but sir, when they get it opened, it could be more and we need you to sign this release [or something] acknowledging your responsibility if the cost exceeds the estimate").
Uh-huh. I took my
watch and went to an old-watch repair guy who did it all for (from memory) ~$200 and it was five years 'till I needed to service it again (and the crystal's still perfect).
That said, my Perregaux was bought second hand and was long since out of warranty, so I did what I did - I am not recommending doing that for expensive watches still in warranty.