What is the cheapest devkit?

The price depends on the model and the seller. Old devkits tend to have beta and alpha games, so they are worth a lot more. Check on eBay and Buyee for devkits and save searches.

 

Totally agree on that.
But it all depends on what you want to do with your devkits.
The cheapest you can buy is the PSP one. Around €150. Though it looks amazing with its wired PSP, no one's really into it. Still my favorite in this range.
Then yes again, 360 and PS2. While the PS2 dev kits are unbreakable, the early 360 ones come with hardware failures, like the big fat extra part separating from the rest of the machine.

Nowadays prices for other systems (Nintendo or the Dreamcast you mentionned) are just prohibitive. Lucky I grabbed some years ago.
Are you interested in the sole machines or are you also looking for the software suites and accessories?

 

Personally I would love a PSP devkit. As for software, I would like a PSP SDK so I could make a game or something like that.
I have a normal PSP-1001 and it would be neat if I could make some sort of 3D platformer for it.

 

I just snagged a PSP one for fairly cheap. Should be here today actually =D
I agree, it's very cool looking. I can't wait to mess around with it

 

I've got 2 and their design is much better than higher priced ones.
Dev wise I don't know how easy it is to grab the dev tools. I got a preview disc with one of them but nothing else :/