Pre-serial 602 had been produced from '59 to late 60's, IMO.

The value(20") seems to me from $150 to $250 on ebay, but it mostly depends on the weight. Recently, no one jumped on Pre-serial 602 20"(2100g-ish) at $175. In addition, one of the indicator of the store price.

Looks pre-serial number, as Yoshi suggests. Serial numbers came in 3rd quarter of 1972 as best I can make out.

This document has 6 years of sales data collected mainly from eBay

http://black.net.nz/cym2011/pcalc2.pdf

I pulled out the last (almost) 2 years of data to have a look at and compare it to the whole time series in order to see if prices had shifted much. They haven't for 602 20" rides.

So $180 (plus or minus $50) for one like that is what I'd expect too get. Preserial ones which go for a top price tend to be mint and/or with the red ink intact. Or they are very light (less than 2000g) or heavy (2500g and over) which have their own special sound.

The $325 price for one at Maxwells is on the high side, but you are paying for excellent service and reliable quality from a prestige drum shop. If I were selling one myself I wouldn't expect to get that much, and if I were buying I wouldn't expect to pay that much. But then I'm a canny buyer.

Hope this helps.