I've seen lefty jagstangs go for $1,100 before they were reissued. After they were reissued by right handed only, I expected them to hit more but I haven't since.
I had a running list of lefty jagstang/jaguar/mustang on my ebay, showing the final prices, but apparently they only stay there so long, so meh.
Anyway, in the past 6 months I saw a beat up blue one go for like 650 , I bought a red one with "buy it now" for 600. In mint condition with a tolex case. It was a 2001 model and the plastic was still on the pickguard. My blue one is in same condition, I think it's a 99 or so.
Let's not forget the Jap mustang that went for 1,250 a few weeks back, you can get them new from Japan for a lot less than that.
Then I bid 600 just mucking around on a blue one, Neitz was the winner for 800 or more, but the seller wouldn't ship to Brazil... the 2nd highest bidder didnt want it, so he sent me a 3rd chance offer , I had 24 hours to decide if I wanted to buy it for 600 or he would relist it. Of course I did it, and then I put my red one on eBay for 600 reserve and 850 buy it now , and it sold within 24 hours.
It was all part of my magical buying selling spree to end last year, I bought 1,150 dollars worth of guitars but sold them for 1,600, once again, I have to say MUAHAHAHA.
Basically, to give you a short answer , depending on the condition of a lefty jagstang and who sees it the 7 days its listed , and how much money they have to spend.
My only advice is have money set aside for it, and if you really want it, bid low and in the last hours bid as high as you possibly can, and try to find one that's banged up or modified... those appeal less to the "collectors" , while at the same time you don't have to worry about dropping it, or changing something. I wish I had a banged up one just to bang on and repaint etc etc.