If it's for an Atlas 618, it'll have a M6-500 or 501 casting number on it. A quick ebay advanced search for "atlas milling attachment" showed prices that vary from $150-$350. Condition and completeness are important to price as are accessories. If it has its vise, screws, vise jaws, draw bar and spacer, milling cutter holder, it'll usually fetch $250 and up.
John