There are many different ways to evaluate the value of your blog. If you think that the businesses in contact with you right now are going to provide value to your constituents, you should consider working on a PPM (pay-per-1000) or PPC (pay-per-click) model to start. In these cases, you can easily fall back on industry averages and then tweak based on supply/demand.
PPM - pay per thousand (where M = roman character for 1000) is impression-based, meaning you'll run their add for a number of times it's shown, not for a period of time.
PPC - pay per click is event-based, meaning you'll run their add until they use up the click events.
In both cases you'll need an ad management system. The choice of which one is going to be dependent on what your blog is currently using as a platform. In both models, you should be getting paid up-front and set up the ads as pre-paid accounts, from which your ad management system will deduct the alloted/agreed costs from.