I pretty much agree with Cody re the Sterling mags, the prices for the 40rd parkerized steel mags are closer to $90 in good condition and over $100 for new.  The more common black alloy 20rd mags are in the $40 to $50 range.  The less common 20 and 30 rd parkerized steel mags usually sell for around $75.  The 20rd black alloy mags were probably mfg in the US by Federal Ordinance for ArmaLite.  The Sterling parkerized steel mags were mfg in England and my guess is that they were intended for use with the select fire Sterling AR18 but also sold as accessory mags for the Sterling AR180.  I have in my collection some 30rd blued steel Sterling mags that I doubt many were mfg.  Seperate 20rd alloy mags and possibly 30rd parkerized steel mags were mfg by Federal Ordinance for ArmaLite for the Costa Mesa and Howa AR18/180 rifles.  At this point I think I have samples of all the mags mfg for the three models of the AR18/180 includeing some rare early test mags.  Kobren.