'warmwaterjack' and 'diving4treasures' are the 2 biggest E-bay sellers I've bought 109/156 or MK5's from, both offer units for sale on an ongoing basis. 'diving4treasures' still had 2 or 3 up for sale as last I looked, ranging between $ 150 - $300 (or best offer), which is still a lot cheaper than Japanese prices. As 'rhwestfall' noted, prices do seem to have climbed a bit, so I'm glad I've already got my stash as well.
When I was shopping, nice condition units 109/156's were in the $ 75 - $ 125 range.....so these days 'cheap' nice condition units are an urban legend....although 'beater' units can certainly be had for much less $. 'Couv' is a great guy and can likely cover your needs without even having to do E-Bay though.
I've even managed to acquire a number of literally new-in-box late generation BA 156's from a dive shop in Germany.
With respect to my 'stash'......well, all I can say is you DEFINITELY will want to attend my estate sale when I pass away!