I think you have to identify what problem you are trying to solve. Your site essentially charges by the number of properties which presumes there is value in them building a site with just their properties.

Do real estate agents have a problem listing their properties?

In the U.S., there are a number of property listing sites and, presumably, there is value in seeing listings from multiple realtors, not just one.

So...

Is your site really solving a problem that realtors are trying to solve?

If not, does open sourcing make this service more valuable?