What is the best cheap CMS or open source which is SEO optimized and where I am able to get the MLS included in the site??
I use Joomla (JUST moved from Mambo today actually) on most of my sites. The new Joomla SEF component is AMAZING. And its free.
If your mls is using the raptor system insertion into just about any of them should be easy. It isn't any more complicated than inserting adsense code. The real choice then becomes what you want the site to look like and do other than the mls insertions.
Check out http:// mambo. open-realty. org/ It is Open realty integrated with joomla/mambo. It should fit your requirements.
Your post was about MLS included. So far everyone has sent you to http://www.open-realty.org That is a fine product but does not address MLS. It allows agents to list properties, but an MLS is all the listings under a Board of Realtors. If you are looking for a script to handle your areas MLS you won't find it. I looked long and hard for one about a year ago. Could not find it then spent 3 weeks writing my own. Every board of realtors have their own rules, the databases are different.
Real Estate Virtual Assistant may be able to help teach you what would be the best option for SEO for your site to add mls to it. She offers all kinds of services to realitor's and is very savy in marketing your business and www.virtualcas.com may be able to help you find the right resources if you want.
You could very easily use Joomla as suggested, and use the open-realty component to embed real estate listing directly into the site