I replaced my Boxford AUD with a modern Boxford and am very pleased with it.
Make sure that it has the headstock bush and changegears. If the one you are looking at has the full 3 lever gearbox then and is in good nick then the price is determined by how much comes with it. A bare machine will be just sub £1000 upto £2000 for one with everything with it, ie bush, gears, chucks, faceplate, catchplate, studyies, QCTP with lots of holders working coolent pump.
I paid £1100 for a 10-30 and then around £400 for all the missing bits. Boxford still sell part but are pricely.
These are very good machines which are under priced compared to other UK machines such as Harrisons and the smaller Colchesters.