At one level, the value of an advertisement is always defined by that old economics chestnut whatever the market will bear. That is, if your site is sufficiently popular with the right demographic that advertisers will pay $10,000/month for a slot, well, that's your proper price point.
That's not easy to figure out, of course, though we can get some basic quantitative data from scrounging around on the Web, including inbound links, PageRank, Alexa rating etc. Those all help you get a picture of how popular a site is and how "authoritative" it is in its own marketplace or niche. A site with low traffic but links from top, highly-respected sites, for example, would have a high PageRank from Google's algorithm, even though its Alexa rating would be low.
Fortunately, however, you saved me the bother by sending me some basic data on your site: "I have RSS feed on it as well as del.icio.us bookmarks. At the moment it has unique visitors, depending of the month, of up to 200 per month."
Based on that, I would say that you are extraordinarily lucky to have a company seeking a link on your site at all: 200 unique visitors per month translates to less than ten per day, and even if your advertiser has the most unbelievably effective advert that's going to garner 10% clickthrough, and they can generate sales to 10% of their visitors, that still means you'll help them sell two products per month max.
Nonetheless, they came to you, so you clearly can charge them. Your PageRank is 2/10, which is quite low, your traffic level is low, and according to a quick Google search, your inbound link count is zero (after this posting, it'll be up to one, at least).
I would suggest that you ask for $50/month for the advertisement and don't be surprised if they come back and offer you $25 or even $20 for a month's worth of visibility on your site. The important thing to remember is that it's always better than zero, and even $25/month across a year can buy you a new lens or other gear, all because you have a Web site. Pretty sweet, I'd say.
Good luck. Let us know what you asked, what they offered and what you settled upon for the advertisement.
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In this kind of situation, the blogger is usually afraid that he'll name a figure that's too high--and then he'll be sure that if he said ten bucks a month less he would've had a customer.
What would be a help would be a voluntary reporting Web site (anonymous) where bloggers who sell ads talk about their traffic and the rates they get. I think more than a few bloggers would be willing to pay for such a site.
--Mike