I'd say 1000 to 2000 assuming it is in good condition and depending on what comes with it.? Steady rests, milling attachments, etc, are valuable.? Atlas lathes seem to have jumped in price over the last few years. ? But price depends a lot on the local market. ?
I recommend finding someone who is familiar with Atlas lathes to go with you for inspection.? It would be easy to spend hundreds more replacing things that are broken or missing.? And a badly worn bed or spindle could turn the lathe into scrap.?
Jon M