I have the D5-XL and I paid about $250 about ten years ago on Craig's List with voltage stabilizer and power supply. The answer is what ever they are selling for and how good a negotiator you are.
I bought one locally for $80 complete with voltage stabilizer, power supply, spare dichroic head (two heads) and 4x5 mixing chamber. However, it's untested. It didn't come with any lens boards or negative carriers.
Depends on the exact scope of supply and on the condition it is in.
If it is rather complete ( i.e. With several lens board, the turret, both mixing chambers, the chromegatrol and the relevant negative holders and masks) and in good working order (no rust, the head moving up an down easily, no vibration on the head from the fan, ...) i would pay a few hundred dollars. If it is incomplete or needs repair, it should be priced accordingly cheaper.
As mentioned the extras mean a lot. Lenses, lens boards, negative carriers are easy to sell and ship. The enlarger itself will depend a lot on the local market. Get details on what is included and the condition and do your best bargaining.
Last month I bought a D5-XL with with Dichroic II head for $10. The CL ad was up for for several weeks with no takers. Came with one lensboard but no lenses or negative carriers. It's a very nice machine which I'd happily pay 300-400 depending on the extras. The real value is in the lenses, carriers, timer, etc you get with the machine. It's a lifetime investment you're making so initial price isn't all that important. Find a good one and use it.
it will be worth more than the price you will have to pay for it. Prices vary quite a lot. As written above, this is a lifetime investment. Get one with many accessoires and never care about it again. I paid for mine EUR 300,- 3 years ago in Germany and nowadays the go for between EUR 500 and EUR 1000,-, depending on the region. Enjoy the Omega!
So much depends on the seller and therefore condition. When was the last time it was successfully used? I have one in my attic with a broken yellow filter; I bet most sellers of enlargers don't really know if theirs are working. Then there's the question of whether the power supply works.
I had the opportunity to buy one last year, asking price was $100. I passed on it. It was in very good condition, but I already have a D-6 condenser head with all the accessories I would ever need/want, and I'm down sizing my photographic load.