I bought a DDR solo for $275 and sold it for $800. It probably was one of the best solo cabinet I've seen in a long time, but it went to a friend; so I don't feel bad. Had the deluxe stage (added pad lights and bar), great cabinet condition, and running 4th; so it included the flash card. I also had a Korean doubles, which I bought for $275 and sold for around $500 or so; since it was a bit of a project. Pricing can vary on Solo depending what mix is on it. The "best" version to find is a 4th solo, since 4th and 4th Plus use the flash card; which is needed to install doubles versions from 3rd - 8th. I believe all of the other mixes don't use the flash card.
The solo pads only have two sensors in the middle of the arrows, while doubles have four around the outer edge of the arrow. The doubles pads will be more responsive, but I don't think most people would notice. They don't really feel any different playing on a solo or doubles. They're both steal, and heavy.
If you're just looking to get one and save on space, a solo is fine. Although, a doubles is nice if you have the room, because of the two player aspect. Also, with doubles, you will be able to run more mixes; with solo it's limited, especially depending if you have the flash card or not. Finding a new flash card to add is a pain, since you have to use a specific one; and they don't pop up all that often, plus they're expensive when they do.
As far as pricing, for a Solo running non 4th, I'd pay around $600 - 700 for a good condition one. Solo running 4th or 4th plus, you're looking at $800 - $1000 on it. For a doubles, I would stay away from Korean ones and the namco cabinets. A good condition US or Japanese cabinet running any mix (8th aka Extreme might run a bit more, since it was the last version to run on the 573 hardware) will run around $1200 - $2000. It really depends on condition, cabinet type, mix, and hardware (memory slot or not).
I wouldn't take my prices as 100% truth, since those are just the range off the top of my head on some that I've seen recently; and from owning and selling a Solo and doubles.
I know someone selling a doubles extreme, japanese cabinet and great condition. Plays great, and really no problems with it. He wants $3500, and it's really overpriced. I think the $2000-ish area is where they start to max out right now. That's Japanese, running Extreme, memory card slots, and no problems game wise and cabinet in decent condition. If it doesn't have those things, start decreasing the price from there.