There are a lot of websites on BuySellAds.com, the dominant ad marketplace for web design related sites. At press time, the most expensive ad cost $4,000 over at Smashing Apps. The cheapest BuySellAds will allow is $10. However, there is a lot of invaluable ad space in terms of impressions they can deliver.
Here is a roundup of inexpensive but effective advertising opportunities as evaluated by number of impressions delivered, quality of brand, placement, size and traffic.