I'm in Illinois, not California, but here's an idea of what bikes can go for around here:
I passed up a '73 that was ridden last less than two years ago... needed some TLC but didn't look all that bad since it would be redone through your cafe project anyway: He was asking $250 but was 2 hours away and I didn't care to drive that far.
Here's a runner that might be good for your cafe project, but again, in Illinois:
http://chambana.craigslist.org/mcy/4814883944.html
Within the last 5 years I've bought:
1. A non-running '75 with clear title for $400... had the wires to the coils backwards so wouldn't start. I switched the wires, rode it for 2 years, and sold it for $650. Was a pretty decent bike.
2. A '75 with clear title that wasn't running for $200. I parted that one out for a profit, but still have the engine (turns freely), exhaust, front end, and a few other misc. parts.
3. A running '75 without title for $400. Parted it out and still have the engine, front end, exhaust, and misc. other parts.
4. A non-running '75 with clear title for $600 ( was on Ebay for $2500 or offer, and I offered $600)... had an Amen Savior frame, Ram Flow air filters, and was actually a pretty nice vintage chopper. Turned out that my legs were way too long for the setup, so I parted it out for a decent profit.
5. A running '72 with clear title for $1,000. For whatever reason, I really wanted this one, but spent way too much for it. Needs a base gasket because it leaks, and has a couple of broken fins on the head. Pretty decent bike, easy project, but I've basically moved on to woodworking projects and haven't done anything with it for 3 years now.
My point is simply that there are a lot of old SOHC 750's sitting around, and you don't HAVE to sped a lot to get them; at least that's my experience sitting here in the midwest. It's easy to find one that you THINK is really close to what you want, but if it's not exactly there yet, you should look very closely at what you need to complete the project. And if it doesn't at least start when you look at it, it needs to be pretty darn nice or have something special before I'd give more than $400 for it... for example I was really after an Amen Savior frame when I bought that non-runner for $600; once I decided I really wouldn't settle for the style, I ended up getting over $800 for the frame alone because it was "special".
In a nutshell, $2500 for that bike you're looking at... is a LOT of money in my opinion. However, it's worth what YOU will pay for it, so there lies your answer. You've seen pictures and have a decent idea what you'll give for it. To me, it appears to be a pile of parts... $500 and probably a little more if the loose parts are exactly what you're after and worth the extra.
Come to Illinois and I'll make you a "deal"... to get that $2500 that's burning a hole in your pocket:
1972 running 750 with clear title... needs base gasket
1971 parts bike, engine in pieces... rolling chassis, but the rest is truly a "pile of parts"
'75 engine that was last started 2 seasons ago
'75 engine that turns freely, but hasn't been started for about 5 years
'73 engine... stuck
New, never installed set of pistons, rings, wrist pins, and head gasket. Oversized (836) "stock" type compression
New set of engine gaskets
Misc "pile of parts".... bring a truck.
