Affiliate Network System

We need a custom built system that does the same job as [login to view URL] but capable of handling literally millions of transactions per day over 20+ merchants, and every customer automatically added to the affiliate/MLM system. AffiliatePro have quoted us for an additional module to extend their functionality to include this, but I am curious as to whether the money would be better spent on a fully custom system. Please read over the specs at the website above and give me a ballpark figure for a ground-up development. Full specs will be hammered out with the winning bidder before the final, formal quote is expected. Note: I have selected a low budget simply because I do not wish to influence bids. Please bid what you think is your best price.