I stumbled across a nicely priced laser printer that features a built-in duplexer. The Panasonic KX-P7105 is a monochrome laser printer rated at 15 pages per minute (single sided), and it is unusual for its class since it has a duplexer. (I'm a big fan of my duplexing laser printer because it saves a lot of paper.)
This printer has a generous 16 MB of built-in memory, so it'll do just fine with complex graphics even at 600 dpi. The printer seems quite compact in size. Driver support under eComStation and OS/2 Warp is excellent, since this is a standard PCL6 printer. (You can use the HP LaserJet 4 or LaserJet 4000 duplex driver for good results, for example.) There are both USB and parallel port interfaces.
Price is $149.99 plus shipping for this new printer from TigerDirect.com. (Look for item number P172-1019 P.) You can get an additional 2% cash rebate if you visit FatCash.com and then click through to TigerDirect from there. That's about $75 less than I've seen it elsewhere, and I believe it is the lowest cost monochrome laser printer with a duplexer.
Check consumables cost. (This printer has separate toner and drum consumables.) Last I checked the per-page printing cost is reasonable, but I recommend double checking that.
This Panasonic web site:
http://catalog2.panasonic.com/webapp/wcs/stores/servlet/...
has details on this printer.
I have no affiliation with any of these companies.