Ad Management Software

I need an easier way in which to manage display advertising across about 100 sites. Most of the advertising sold will be basic advertisements (it is very rare Google AdSense or other programs will be used unless we are unable to fill up space, which is very rare)

At the moment I am placing advertisements individually on each website. It is a LOT of hassle (takes about one day each month to just change up the advertisements)

I am looking for a way in which this can be done centrally. I want an easy to use piece of software which will enable me to manage the display advertisements on each website without needing to login into them each month. I want to be able to paste a single piece of code into a website and be done with it (i.e. I can change all the adverts from my central control panel). It also needs to be able to record the following:

• Clicks
• Impressions
• Click-Through Rate (not 100% Necessary with the previous data, but could help)

I also need there to be ways in which I can manage the adverts whilst allowing clients to view how their adverts are performing, or perhaps for them to order new advertising space.

The software must be free, or as close to it as possible (sub-$49). I am not trying to be cheap there. It is just that if I can't find a decent piece of software (customizable) at a low price then I may as well just code the site myself and have my own custom solution. The idea of looking for a piece of software like this is to save time.

I did not have a way of managing adverts in the past simply because that side of the business grew far too fast. More and people are now getting in touch with me to sell their banner advertising (up until about about three months ago I managed just 20 sites, now there are 104)

Any suggestions?

Thanks!