Tom,
The venturi deals are, I think, sold more for veneer press application than anything else. Consider also that they are (usually) quite loud, and net, net expensive since you've got this big compressor running.
If you want the cheapest solution, go to several home appliance repair shops and snag 1 or 2 FREE compressors that have been removed from dehumidifiers. After the shop removes the freon, the units are just so much scrap metal and they're usually glad to give you the compressors for free. It's then pretty simple to wire them up and do a bit of flare-fitting plumbing to hook them up to your lathe. I use 2 such units on my Stubby. They'll pull 27-28" of Hg. and move about 5-6 CFM in free-flow mode.
Note of caution: Also snag one of the fans from the units to use as a cooling fan for the compressor (s). These things have an internal thermal overload and will shut down without warning if not kept reasonably cool. The little fans in the appliance work just fine for this and, like the "pump" are free, and the thing will run almost silently all day long.
Mark Mandell