I got Bugeye fever about four years ago, so I started looking on Ebay and Cars.com. One came up for sale in Knoxville that was in good shape, but I was outbid as it finished at about $7500. A few days later one came up with a Buy It Now price of $6500. I spoke with the owner on the phone, who assured me it was rust free, straight, running, but not stopping. I bought it without looking at it.
It turned out to be a very, very sound car. Since purchase I had it re-painted (which confirmed the no rust diagnosis), new upholstery, new brakes/MC, some new gauges, a rebuilt trans, and several other things. I did almost all of this myself, and the paint job was done by a friend (for no charge but I swapped him some FF miles/hotel points so his employee could take a Hawaiian vacation). So my big cost has been materials, which are all surprisingly affordable on a car very easy to work on. Still, out-of-pocket ran about $3K over 2-3 years, plus an entire 18 months the car didn't go anywhere.
I think there is a pretty straight trade off between up front price and later out-of-pocket costs. For $6500 plus about $500 (tires/MC rebuild/trans/brakes) I could have easily been on the road. I just went a little further to get it "my way." I'm still working on it (1275/sway bar/3:9 diff), but that's all discretionary.
Bottom line: I don't think you can get into a BE workable BE (not eaten up by rust) for under $4k, but for about $10K you could have a very, very good vehicle.
Mike Pennell