You need to be very active if you are seeking to find a low priced coupe. I found my car 15 years ago with a low budget, high effort approach.
Buying from a dealer and the auctions is out of the question - they will not price a good coupe that low.
Obviously, you need to review and follow every coupe for sale that is discussed here. Most likely you need to get into a private party transaction, and one where the car hasn't been on the market very long (if at all). I suggest attending events where coupe owners bring their cars - Vintage at Saratoga comes to mind - and talking to all the coupe owners you meet there. Do they know of cars that might be coming up for sale? Then I would get to know the shops in the NE that work on old BMWs and talk to the owners there. Start with the ones that have worked on your BMWs and then work through the coupe owners you meet at car events to find others. And I would find a way to get an introduction to Mario Langston at VSR. He's well known for his coupe work, and has a good client base. He evaluated a car for me that I didn't buy. Let those guys know you may be a buyer if one of their customers wants to sell.
I did all of this, and it took me three years to find my car. The car had good bones, but it had been sitting for at least 5 years. And it cost more than your budget. The car had a new battery and new tires, but I still needed to repair a bunch of electrical stuff, replace all the hoses, belts and fluids, redo the brakes and a handful of suspension components and fix a vacuum leak in the brake booster hose to make the car driveable. I did all this work myself; if you can't do this you need to budget for the fact that a low priced coupe is going to need work right away.
Maybe the market has gotten a lot softer, but I suspect that you may have to consider a larger budget to find a coupe with good bones and then make it driveable.