ebay.co.uk
or gumtree in your city.
PMG5's come up every day on ebay, I just missed one today with some hectic bidding in the last minute (from £80 to £216 in the last 60 seconds).
sub-2GHz single processor G5's go for about £200, 2GHz and above single processor for about £260, dual processor for about £320-£360.
PBG4's in the same place, but no idea of prices - I had one already.
Having the original discs doesn't seem to make any difference to the price.