if it's monthly, I think it's a good deal for the buyer (I would charge at least $50-60 though for this kind of traffic). If it's one time payment it's way too low, try $200 or $300 but then you'll have to make sure that the link stays there indefinitely, so it's still better to charge less but do it recurringly...
GO for 100 USD a month mate. Still people would be interested if the visitors and relevant to them
I think $30 per month is okay for link on the side but keep in mind to charging different prices depending on how popular the website is and how many audience visit. Tell everyone about your 90K visitors ask for a more money like $40-50 for month also give discounts to those who are working with you from long time. Share details about your website. What is it for and how much attention it could bring to client's brand not just how popular it is.
