There may be software out there that will do this for you. With my personal sites, I have built in software that allows me to configure my links to track click throughs. Depending upon how you go about it, you may have to manually create reports each month to send to each advertiser. But unless advertisers can monitor the reporting for themselves, they may not like the manual approach. It's best if you could find something that can do all this for you and allow advertisers to view the reports too.
Here's something I found that may help, but I have not used it myself: