It's a multiple hours job AFAIK, so I'd guess depending on labor rates in your area, $1K or more.
Since the leak is slow, have you tried high mileage oil with the "seal conditioners" (aka seal sweller additive), something like Valvoline Maxlife ? It will take a while to work, and you may always need to keep using high mileage oil, and it might even leak a little more at first to have new oil in with fresh detergent that cleans away some sludge if present. What grade of oil are you using? If it's 5W-20 you might switch to 5W-30. Thicker oil leaks less. You might even run 10W-40 in summer.
When your mechanic told you it wasn't worth it, what was his estimate? If you are unfamiliar with other shops in the area, you might go a google search for automotive repair near me, and it should provide google customer reviews along with the search hits. Make a short list and call them asking for an estimate. Many of them may want you to bring the vehicle in. I'd still call around to get as many ballpark estimates as possible before taking the vehicle to multiple places.
Whether it's worth it depends on a few factors, like what condition the vehicle is in, how many miles and on which engine, whether you like the vehicle or not. If it has the 4.0L SOHC engine, and past 150K mi, it is the least desirable combo to spend more money on, except if the engine is pulled you could have the timing chains, guides and tensioners replaced while they're at it, and that fixes that issue. I would do that if the vehicle is in good condition including the undercarriage/etc not rusted out, but ultimately the cost to keep it going and whether it is worth it on a vehicle that age, can depend a lot on whether you can DIY most of the lesser repairs. For example my '98 is rusting out and needed new hard brake lines. DIY was under $100 including new soft hoses. A shop would probably charge over $1K.
If it's in good condition, and considering you can't get much of anything to replace it for the cost of the rear main seal job, then it may be worth having it fixed if it gets worse, but as RangerX mentioned, many people just live with a leak until it gets bad enough to be a problem for whichever reason.
I recall someone on some forum, had to have a leak fixed because his employer was pissy about leaving oil in their parking lot space. Some people suggested throwing a piece of cardboard under the vehicle.