Anyone considering the purchase of a 240sx convertible would be wise to read this sticky thread:
240sx-convertible-buyer-s-guide-t312662.html
It's a fantastic resource with tons of info and pictures on things to watch for when buying a 'vert.
Anyhow... Mr. E. Fish above (inside joke) was right.. it's hard to give you specific advice on this car you are considering without any additional info. If it's clean and has no major issues, has a top and rear window in good condition, no transmission issues, and a decent interior, then yes, a 'vert with 120k miles should be around $3200.. that seems very fair. Heck i've seen really clean ones go for $4k or more pretty easy with around 120k. But most are no longer "really clean". These cars are aging, and most have issues that take money to remedy.
Good luck with your search. Feel free to post pictures and more info if you want better advice.