Advertiser wants to buy site wide link for 3 months

How much should I ask for a site wide under navbar 10 word advertisement that links to the advertisers website with two different words?

My site stats as follows:

Daily visitors 1100-1500
Homepage PR: PR5
Site Age: 1 year 4 months
Approx. total posts: 30K

The advertiser offered $50/month for 3 months. Should I ask for more?

Do you currently run AdSence on your site?

If so I would look at your average income and CPR from that over a month and use it as a guideline of what you would expect to be getting from this advertiser.

Ceri

Yep, I run adsense though I earn more from ebay. I'll have to consider the earnings from that. What's nice is, I'll get to continue displaying my other ads while placing their ad just above the adsense ads. Thanks for the feedback. Any other advice is welcome.

Hard question

I got 150 EUR/month for 1 link: 2 words link, 3 words without link
link was in the sidebar below the navigation

daily visitors: at this time 10'000-15'000
PR: highest 4
age: 8 years

but I do not sell links anymore..

I would advise highly AGAINST selling text links. Google is against them and you will likely suffer ranking consequences.

How do you differentiate selling advertising vs selling links? I assume selling advertising directly without using a 3rd party broker such as Adsense or Doubleclick is ok, right?

Paid links - Webmasters/Site owners Help

Someone offered me a $400 donation to display the following on my homepage for 6 months.

Testimonial
Something very interesting has happened. We’ve been awarded by the guide to <a href="site url">poker promotions</a>, siteurl.com. Their content department is running this campaign to help out gaming resources like ours. The focus point for their work is to help out projects that focus on serious and educational <a href="Articles - Gamerz Needs - For All Your Gaming Needs!">articles</a> that strive to help out the visitor, which is something that we hold dearly.

Is that something that google would look as bad or is that fine?

I would suggest nofollow'ing it