Saw this on Farnell.
Yours for £79.99.
Looks pretty good for that moolah.
Requires a spare PC of some kind to run it, but I imagine most people have an old computer knocking around that would be up to the job.
(http://uk.farnell.com/productimages/large/en_GB/2432906-40.jpg)
(http://www.element14.com/community/dtss-images/uploads/devtool/diagram/large/24120ba34f60d12fe4b557a01aeae7cf.png)
http://uk.farnell.com/bitscope/bitscope-micro/oscilloscope-2-6ch-20mhz-40msps/dp/2432906 (http://uk.farnell.com/bitscope/bitscope-micro/oscilloscope-2-6ch-20mhz-40msps/dp/2432906)
that's really cool. The second pcb on the pictures are a raspberry Pi B+. As it happens, I have one of those sitting around. Maybe something to consider