I just bought a pair of 620 can's to modify and paid $50 for them . They were on the bike for less than a week before the guy replaced them . So they look better than the ones I have on the bike now so I just might modify mine and keep these for later.
I picked up a pair for $22US + shipping on ebay (shipping was as much as the pipes). Not many people want them so they go for pretty cheap. I bought them and had them sent to Strati to be improved.... My recomendation is to either keep them (you may need them for a safety when you sell the bike or some future smog inspection who knows, that is why I did not modify the ones that came with my bike) or have him give you a price and see what he will pay. Odds are you aren't going to get rich selling them. Maybe buy a couple of tanks of gas. If there is no shipping involved somewhere around $50 to $70 is a good price for both of you.