How much to charge for Real Estate listings?

[Andy]

I am shortly going to launch a Real Estate site which lists properties for sale with a thumbnail image. Clicking on the title of a listing will take you to a details page for the property with a contact form and a gallery of photos. The photos will pop up into a large image when clicked on.

It's actually for Spanish holiday/retirement homes marketed by/to the British. I was thinking of charging say £20 (~$40) per month per listing. My initial target will be to get 100 properties listed.

Also, there will be a free discussion forum for issues regarding Spanish property and living in Spain.

For traffic I will be highly optimizing the pages for keywords, posting to the forum and seeking out related sites to build inbound links from.

Given that a property will be valued at say $200,000 what do you think is a reasonable monthly listing fee?

I have seen a rival site charging $20 per month but they are not specialized on Spain, so I think with the forum and having a better photo gallery etc. I can charge more.

Any tips appreciated, especially if you have experience of advertising or searching for property online.

Further monetization will come later from real estate service companies that may want to advertise on the site.

Thinking about this further 0.1% of $200K is $200 so this seems reasonable to me. What do you think?