I agree that a good working combine could not be had in a lot of states for $750.
However in other states the older smaller rigs get pushed aside as farm sizes increase, and the old small farm machines can be gotten quite reasonable.
I bought such a machine for $800. I replaced a wooden reel arm and cut my entire first crop without anything more than setting the machine. The combine was about 35 years old, but kept up in good shape.
Combines are a fairly complex machine. Depending upon the machine you may want to buy it to use, or play around with the components such as engine, hydraulics, transmission, etc. Expensive as a toy, but you might find several machines could be built out of the one.
Combine engines are often hooked to generators. The engines and hydraulics are often used in log splitters. If you are handy the sieve and fan could be made into a machine to clean crops. (But of course the entire combine is made to do that.) The grain bin could be put on supports, a cover added, and you could buy bulk feed for your animals. Well, if a local dealer delivers. Put it on wheels to take to the dealer to buy there and save delivery cost. How about using the cleaning fan to blow fresh air into your shop during the hot summer?
Perhaps you could use the frame, engine, transmission, etc. to build a tractor. The hydraulic lift cylinders, etc. could be used in making a loader. If the combine has a cab you could even have a tractor with a cab on it. Perhaps the hydraulics could power a homemade 3 point lift so that implements could be used. How about cutting down the header and making it into a sicklebar mower to cut your acreage? Perhaps remove the feeder housing so that the header cuts hay and augers it into a windrow to be picked up after drying.
Lots of possibilities if you are handy, have a shop, welders, etc. Dream and think, then set to work.