As far as I know, the main reasons DDR2 is commanding a premium comes from 1) still in demand and, 2) not being made anymore. Really a guess, not based on anything I've read.
The differences are summed up with this statement by Matthew Murray in PCMag.com
"...followed, in 2003, by DDR2, which refined the idea even further with an internal clock running at half the speed of the data bus; this meant it was about twice as fast as the original DDR (200-533MHz, with transfer rates up to 1,066MTps), but again used less power (1.8 volts). Naturally, DDR3 was next out of the gate (it debuted around 2007), with its internal clock cut in half again, its speed about twice that of DDR2 (400-1,066MHz, for a maximum transfer rate of 2,133MTps), and power usage reduced even more over its predecessor (to 1.5 volts).
(You may have already surmised the next logical step in memory technology. Indeed, DDR4 is already in development, and will probably begin appearing in consumer products around 2014, with wider adoption to follow gradually.It's expected to offer transfer rates of up to 4,266MTps, with voltage ranging from 1.05 to 1.2 volts.)
What's the down side to this constant improvement of memory? Unfortunately, you can't benefit from most of these advances without significantly upgrading—if not outright replacing—your current hardware: A DIMM that uses one kind of DDR interface will not work in a motherboard designed for another. Each type of memory is electrically incompatible with the others, starting with the number of pins on a chip (DDR desktop-style DIMMs have 184, and DDR2 and DDR3 each have 240), and DIMMs using each are keyed (or notched) differently so they can't even fit in the wrong kind of socket. It's therefore crucial that your existing hardware and the memory you want to add are of the same DDR type."
Found here
http://www.pcmag.com/article2/0,2817,2400801,00.asp
If this was started as a discussion thread there is no 'best answer', but you can always "thumbs up" an answer. Select as best answer is rather big and blue at the bottom of each of my replies.