It depends on your client that what kinda budge he may have for his link, but as u mentioned that 30k-40k unique visitors , that will definitely count for a big profit.
depends on NICHE .. Place where You wanna put LINKS ... Source of Your traffc !!! and more more ..PM me url I'll send You price
perhaps you could get this vital info from advertisers themselves. Submit it to sites like linkworth or text link ads and put your price you think in your opinion your site is worth and see whether it sells out. If after a month or two, nothing happens, then you should start adjusting things, lowering the price perhaps and until you get people buy it, you would then know that's a price people are willing to pay. For a 10k-20k pageviews per month, $15 is a good price per link.
Niche, source of traffic, and link location is key to dertermining value. For example: Niche: Funny Videos, Traffic Source: Social Marketing, Link Location: Footer Value: A few dollars per month vs. Niche: Investing, Traffic Source: Search, Link Location: Prominent Value: Hundreds of Dollars per month