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So far all advertisers have sent payment instantly once they are happy with their ad. I'd not offer 30 day payment terms on advertising - that would be open to abuse.
My rates (for sites between 1000 and 5000 page views a day, PR3 or 4 homepages, reasonably targeted to the advertiser) are £30/month for a 125*125 prominent on the home page, and £10 per month for the same on any other page. Mine pass pagerank as I've not yet figured out the technology to do otherwise, and the advertisers seem to prefer it.
I've been charging $60 annually to run a small text link ad on an inner page (PR 3). Pricing it was only a wild guess.
A few advertisers have written back and, surely inadvertently, mentioned a price higher than I quoted. One fairly large site offered more money for a six month renewal for an ad running on the home page (PR5) than what they paid for the ad the entire previous year (new company contact).
So I think I may be charging too little. However, trying to find information on what to ask is difficult. Of course, I know there are many factors.
But it does sound like you have good evidence that your prices are far too low.
One useful technique is to ask a little higher that you would ever expect to get. If the advertiser disagrees tell them you'll get back to them within a week as there's someone else who might take the spot for that price. In my case this was the truth.
I'd love to know what you end up with. But I realise it is beyond the rules of this esteemed forum to suggest you send a message. So I won't.
There are a couple of sites I don't allow direct advertising on.
@vordmeister, thank you for reading the TOS, and casually suggesting that someone NOT send you a private message. That was cheeky :)
Personally I would like to know what factors are involved in pricing an ad placement. Monthly unique visitors, for sure. Content Targeting, certainly. Passing along PageRank, yeah - even though Google hates that. Is it possible to reduce all that to an equation?