Don't forget... there are a couple specific kinds of the high-end Fender "Custom Shop" builds...
Masterbuilt is built by one person who sends you picture updates and emails and who calls you on the phone to ask specific questions and talk about options, etc... they also read you bedtime stories every night and send strippers to your house for your birthday. Ok, not that... but there is a LOT of care and attention involved. The guys who are hired as master builders are some of the best in the industry.
My quote for a lefty "70's style" 5-string Jazz bass was $5500.
The second category is "Team Built." These guys are Fender's "second tier" (for lack of a better term) craftsmen, and most of them are in training to eventually be "Master Builders" or eventually do their own thing. They usually do special limited runs and some of the very small run pieces like the Jaco Tribute bass and that kind of thing.
Team built Custom Shop basses typically sell new in the $3000-$4500 range.
Both Custom Shop basses I have had were team built, and they both kick major ass... so, it is possible to get a bad-ass lefty Fender Custom Shop P-Bass in the $3000 range, but your problem is going to be finding one.
Even though a few have popped up on the board recently, lefty team built Custom Shop basses are usually quite rare.
I know Craig Brody at Guitar Broker does limited run Team Built basses... I think he orders 15 at a time, and he has ordered one lefty jazz bass (it's the one I just sold here on the board). He has some BEAUTIFUL '59 P-Bass relics coming in right now, but all of them seem to be right-handed.
Check this one out...
They're also $4250, so that might be out of your range...
Anyway, if you really, really want one bad, you should call him up and see if he ordered a lefty, or see what it would cost to get you one.