Custom adserver

After trying openx and other adserving systems I think I am better off building one tailor made for the needs of my website. The system is pretty simple: We have 4-6 advertisers. We have 4-6 zones where the banners are displayed Each advertiser has banners. We show on each page one banner per advertiser (not two banners from one advertiser) We split the views evenly (if 6 each one gets 16.66 per cent etc) Example: We have in one page: 728x90, 250x250-A, 250x250-B, 468x60 (=4 zones) We have ADV1, ADV2, ADV3, ADV4 (=4 advertisers) Each one has all 3 banner sizes. We rotate the banners in each position, we show one banner of each advertiser per page. Someone might think, why not use openx and built own server? Well openx does all the above but loads the ads with [login to view URL] and there is no way to create a clean, asynchronous way to serve the ads. That said, we need the ads to be loaded asynchronously! Let me know if you have similar experience and what the estimated cost is. Thanks