I don't know that I've seen a lemon one sell before but I did sell an Olive one about a year ago for nearly $300. I think Olive is a bit more rare than the Lemon. The more common colors look to go for around $20. I'm guessing $50 - $100 but it's only a guess.
I have no firsthand knowledge of these cars, but a quick trip to MEV's T Jet shrine says a Lemon indy is equal in rarity to a gray one. These are one step up from the olive one!!! http://www.tjets.com/ Go to T jet shrine and click on indy for the rundown.. You have a rare piece according to that. I don't know how it'll rank in Bob Beer's book..