That's a shame, still offer stands I go to most of them.
With regard to costs of Planet and Dye markers they are comparable on the second hand market but you will Tend to find that Dye markers are slightly cheaper than Planet ones, when comparing the DM and the Geo platforms as they are both spool valve.
It's like anything the older they are the less they are generally worth, however some of the earlier Geo's such as GSL still command big money as they were limited edition and came with full barrel kits and core IV bolts as standard. An earlier Geo (2.1) could be had for under £300 but this will be around 4 years old. In comparison to a 2-3 year old DM can be had for as little as £250, dependant on condition. I believe a DM15 recently sold on here for £325 which in my opinion is an absolute bargain. A DM will shoot just as well as a Geo and in some cases better.
The later Geo's will be quite a bit more money such as the CS1 which seem to be fetching around £750 and the Geo 3.5 £600 -£700 mark. By means of comparison an Ego 11 will probably set you back in the reigion of £350 and an LV1 maybe twice that. A second hand Dye M2 (the new version of the DM platform) will set you back about £800 and are absolutely lovely but this is where personal preference comes in to play with regard to ergonomics, design and the overall look and feel of the marker.
Both platforms are rock solid and proven spool designs, used by the top teams. One major advantage of the DM platform is that since the DM9 nearly all Major components are common, this gives great spares availability and reasonable costs.
I can only speak from personal experience but my DM's have been absolutely bullet proof. I have only had to clean and lube them and they just go on and on. The only thing I have had to change was an eye pipe on the 14 and a reg seat on the 13.
A DM9 has just sold on the classifieds for £120!! That is an awful lot of marker for the money!