I run a pretty small content site about a hobby, this is in a relatively high cost accessories niche. Product prices in this niche range from 200 - 1500 USD.
The site has the number one spot in google for a lot of relevant keywords, ranking higher than the actual shops selling these products.
I was contacted by someone who operates such a shop asking if he could advertise on my site. I have no idea what I can charge. I could provide details about how many visitors / pageloads I get but then he does not know that and perhaps doesn’t need to?
I realize this is probably very hard to answer but … what could I charge for a 125 x 125 ad block on all 15 pages of my site?