How much is a VINTAGE PEAVEY TKO 75 B AMP worth??

I have a friend who is selling their old amp (he's used it for 10 yrs), I have been l@@king to buy my son an amp with a little more ummph and I know this amp sounds great, the knobs are a little scratchy when you turn them,and the ground plug has been broken off, and some cosmetic wear, but that's all I find to be wrong with it considering what you pay for a brand new amp of this magnitude, and then it breaking in 6-9 months down the road, so HOW MUCH IS TOO MUCH, or should I even be considering it??