System architecture and methods for enabling a client-side real-time auction of advertising inventory that works in conjunction with ad serving technologies. The method according to some embodiments of the present invention comprise: (i) an end-user visiting a web page wherein multiple advertisements are displayed, (ii) for each ad unit on the page, multiple parallel requests are sent from the end-user's browser client to multiple real-time bidders who respond with a bid & advertisement for each unit, (iii) the bids are compared within the end-user's browser and the winning bid is sent to an ad serving system to be compared with other statically priced advertisements and exchange demand to determine the winning advertisements that will be displayed to the end-user and (iv) data is aggregated for each bid and price limits are set based on the aggregations.