£100 a month? What kind of server do they need? Seriously.
So, while you still know everything, How do you propose that the service could be at a lower cost?
Well, all of these are cheapear: http://www.blacknightvps.com/vps-starter.html
Clearly you have not used the toolbar, or have no idea of the scale of the project. The basic package won't cut it.
Property bee http://www.property-bee.com
is a toolbar that runs on Firefox, this scrapes the current price data for a property, sends this to a server, the server responds with the history, the toolbar works with daft.ie and most uk property sites.
Without tools like this we are at the mercy of estate agents, as their knowledge of the marketplace is part of their power, as I plan to buy next year I am happy to contribute to the annual £580 server hosting costs.