It's easy to overlook costs - the main cost will not be for actually making the website, but for maintaining it. By maintaining it, I mean, actually having useful data in the website. The whole trillion dollar stimulus will generate humongous amounts of data which will have to be processed so that we can make sense of it. Collecting it from all the various agencies around the country, processing it and then putting it up on the website will require a lot of employees and that's where the $18 million tag for 5 years comes from. Don't underestimate the cost of data collection.
I could code up a website that looks like Google Maps pretty easily, but it would take a lot of money and time to get all the data that actually makes it useful.