Shortbed chassis value??

[Myself]

I'm going to buy another shortbed chassis to start restoring my wife's '68. One reason is because hers is tweaked some anyway. The other is so she can keep driving it while I get her an entire frame, cab, and parts of the front end restored and ready to go. I have found a couple good straight ones nearby. One is a '72 missing a spindle but everything else is there including the rear end. And it's all nice with only minor surface rust. The other is a complete 70 shortbed with a straight six and utility bed on it. I don't want to pay very much for either one but would like to know HOW MUCH IS TOO MUCH?? Should I spend $100,$200,..$400 ?? What do you guys think??