Custom Blog Ads Rates?

In blatant defiance of all known precedent, someone has not only read my climbing/mountaineering/great-outdoors-in-general blog, but actually thinks it's worth advertising on!

They emailed me to ask how much I'd charge per month for text ads - trouble is, I don't really know, I, like 99% of the rest of the blogosphere, just shoved AdSense on there and forgot about it. Like, really forgot about it, I haven't even had my first Google cheque yet.

So, er.... hmmm.... I don't want to undersell myself, but equally I don't want to give them a price that instantly marks me out as a no-hoper. Ponder, ponder, stick finger in the air...

But I'm a physicist by training, dammit, I should be able to work this out, surely. So let's do a quick calculation.....

It's a pretty low-traffic blog (a couple of hundred hits a day), but it's quite targetted, and AdSense shows an effective CPM of around $3 with a clickthrough ratio of just over 1%.

So, if I'm getting H hits per day, and the CPM (cost per thousand, I think?) is C, then the price per month should be (avg number of days in a month) * (H / 1000) * C, right?

That gives me a baseline price of around $30 / month. Obviously I can adjust that depending on page placement, as ads just below the banner get more clicks than in-page, but does that sound about right for a low-traffic, specialist enthusiast blog?