For anyone to assess the value, you'll need to check the mileage. It seems most Eagles anymore fall into (2) categories, creampuff with under 40K miles driven by the stereotypical little old lady or gentleman, or a completely clapped-out 200K+ Eagle that's practically been run into the ground.
If you're in the first category with a rust-free, low-mileage car, you COULD be looking at roughly $7-9K, pending a check on the transmission, which would probably lower the price a couple thousand if it needs another transmission.
If you're looking at a 200K+ miles, rusty, worn-out beast, it's probably worth about $1000-1500 on the high side, mainly because it's going to probably become someone's parts car for all the Eagle-specific stuff that can't be found elsewhere.
Rust is the worst enemy for these cars, and depending on where you're located and if the car was local to that area, that could really seal the deal one way or the other. If you're looking at an East-of-the-Mississippi, North of the Mason-Dixon ride, it's probably rusted pretty extensively. If you're looking at something from the South or NW where road salt isn't used, you may be looking at a better car with less rust.
Check the front unibody "frame" rails that you can easily see inside the front wheel wells. Any rust here, or "rust-jacking" where you can see the frame steel bulging oddly anywhere because of rust expanding underneath other layers of metal, is probably enough to disqualify the car from buying.
Other areas include the outer rocker panels, where the steel is hidden by the plastic rocker cladding, and both front and rear leaf spring mounts for the rear axle.
I would caution against watching too many of the Eagle "revivals" one can find on Youtube, and getting it in your head that you can turn the key and drive this thing safety home over 500 miles after it's been sitting for 20 years.
Parts are available, but prices are steadily rising, just like everything else. Mechanical parts are the easiest to find, with RockAuto being pretty helpful if you don't want to pay the crazy premiums that the auto parts stores charge. Eagle-specific interior parts are getting harder and harder to come by, particularly anything plastic trim related. One can get lucky sourcing these odds/ends with patience, the forum here, eBay and generally more money than you'd think some silly little piece of plastic could ever be worth.
Specifically about the trans slipping, I would verify that the kickdown rod that goes between the throttle linkage and transmission is present. If it's not there, the transmission will act like it's slipping while driving because it has no idea what it's supposed to be doing relative to the throttle position. If the transmission is legit slipping, forget the Lucas trans-slip miracle stuff. There are really no miracles to be had, the transmission will probably need rebuilt.
If you can provide any photos, that would probably help everyone on here chime in with more thoughts. Good luck!
David