How much would you spend for an ad on a website's page making 100 visits a day, which PR is about 4 or 5...?
You pay on a monthly basis.
The website belongs to the same category.

I guess this is the right thread to post such question.

Not more than $3 or $5 a month, to be honest. 100 visitors is not very much at all.

Thanks for the reply, anyone else?
100 is not much, I agree, it's just an example.
Did you set the price according to your experience in this domain?

I normally base it on a combination of traffic and PR. If you are looking for link building you might be able to get a few bucks more. You would then sell links at an SEO site like Digital Point. Now if a advertiser is looking for traffic, 100 users a day, and that is not even uniques a day is very small. I would guess a link would get 10 click throughs a month at the most. So that value is not very high.

Well, I input 100 (in my mind unique) as an example and if the link is at a good place according to the content of a page, you could get 10 clicks a day even with this amount of visits.

I guess I would go for no more than 3-5$ too

I doubt most people would even pay $3-5 per month. Unless they were strictly looking for a PR boost. Those type of advertising customers are only going to be around until they get the PR boost

Well if it was just for a taxtlink i would pay $5 - $10 for a PR5 and $5 - $50 for a PR6 depending where is ranks in search engines aswell

banner i would pay for 100 Unique $3-5