My first response is why fix em at all? I had an old eclipse bucket boat with nylon military valves and they always leaked a little...so did the rubber from a few spots. It was just understood you'd have to inflate her in the morning, maybe once during the day if a tight boat was important to me at the time... If they don't leak to the point of being annoying I'd leave it for the future buyer, they may be happy as a lark adding air every now and again. You do run the risk of f'ing up the boot, especially if it's already brittle. IMO if it aint totally broken on a poverty boat, don't fix it...and I've had a few!
edit... price... that is really dependent on supply, demand and other market considerations. It sounds reasonable, but maybe a bit high?
Yesterday's gone on down the river and you can't get it back. - Agustus McCrae