The value of a /5 really depends on the condition and any recent work/upgrades.
A very clean-looking but untouched /5 will require several thousand dollars of work, if not right away, then very shortly.
I would look for one that has been throughly mechanically updated, by someone that knows what they are doing. In that sense I don't think $4500-$5k is out of line. However I wouldn't pay much more than $2500-$3k for a project bike, as they get pricey really quickly.
A tip to tail rejuvenation on my 1971 /5 cost thousands of dollars in PARTS ALONE, not to mention labour and time.