First a note, your wording is incorrect, "least cost effective" means the most inefficient, the proper wording is most cost effective.
Photography web sites are a hard thing to promote. The first step is to ensure that your website has data built into it so if people search for terms related to you (for example if you are in chicago and people searched for "photography chicago" your site would come up). A site that is well built will have this in place.
A great free way to promote your photography web site is by using social networking, ie. facebook, twitter or a blog that you can directly engage people with. People will be able to see you work as it occurs and therefore they have current and interesting information to look at. You can do reviews of others' work, or post interesting links or talk about your creative process. Basically post anything that will be interesting to read for people who are not photographers.
A paid service that a friend I knew had great success with was wedj.com . It is for event planning but you can put your business on there and anyone who searches for a photographer will find you in your respective area.
Lastly, TALK ABOUT IT! Post links to your page, have conversations with people when you're out getting groceries or at your other job or wherever you are. Being a photographer is really hard work and you never have an off day. You should always be shooting, you should always be meeting and talking to new people that will in turn be possible clients.
enjoy!
happy shooting