I got mine via ebay-germany for about 250€ i think, which is an okay price. They are very stable and nice instruments, but sadly lack the interesting 7. digit via GPIB without hacking it. Maybe a 34401A is more up your alley DC1MC, they have 7 Digits via GPIB, can be very linear (0.2ppm) and cost about 300-400€ used. Also they appear very often on ebay.de.