I'm not going to say that it would be impossible to do, but it would be expensive and a big headache.
BUT....back in the 80's, schwinn made a 10 speed cruiser called the suburban, that did have an optional freewheeling set of cranks. (see pic.)
This would give you the freewheeling cranks and 2 sprockts that you'd need.
next, you'd have to make the jackshaft and figure out what size sprockets to put on it. the shaft itself would need bearings (at least 2 quality bearings)and you'd have to make a mounting bracket. you'd have to spend some time with the mount because the sprocket alignment would have to be dead on.
from there, it would be a peice of cake, because then all you'd have to do is run the engine chain to the jack shaft, the jack shaft chain to the pedal sprocket, and then the chain from the pedal sprocket to the rear wheel.
I'm not sure what the double 10 speed sprocket would give you as far as gear ratio tho...I have no idea if the sbp kits have 2 identical sprockets on the cranks, or if one is smaller than the other. you'd have to research gear ratios to see what you would have if you ran the suburban sprocket as it is.
I think the hardest / most expensive part would be making (or having a machine shop make) the jackshaft. you could probably get away with welding 2 sprockets to the shaft, and then running 2 bearings on the shaft itself. but that would cause maintanance problems. if one of the bearings ever needed to be replaced, you'd have to cut it poff the shaft.
There's probably a way to do it to avoid that, I'm just throwing things out there off the top of head.
This whole idea is just a shot in the dark, but i'm with you...$250.00 is a lot of money, but as others have said, they've taken all of the guess work out of it.
I mean if you really wanted to try it, i think the suburban sprocket and cranks would be a good start.
you'd probably have less than $250.00 in making it, but the big question is....will it work when you're done?
here's a couple of sets of the cranks with sprockets.
http://www.ebay.com/itm/SUBURBAN-FF...707?pt=LH_DefaultDomain_0&hash=item58921a3b53
http://www.ebay.com/itm/SCHWINN-CRA...599?pt=LH_DefaultDomain_0&hash=item1c2572b86f