Work the Fringes and the "Long Tail":
Note that there is a long list of terms that derive from the basic popular ones and have more words in them that limit the results. That's called the "long tail". By choosing those that will work for your niche and site, one can spend less due to the lesser competition. There are frequently a great many terms that fit this description.
Look for Search Terms that Fit Your Market but are Less Popular:
Take for example denver luxury real estate, which had 185 searches in one month. Note that top positioning in this term is around $2.00/click. That's quite a savings and very focused on your niche if you're working the luxury market.
If you're working downtown lofts, the "downtown denver real estate" term, with 163 uses, could be a gold mine at a bit over a dollar a click.
Run the Numbers for a Great Cost-to-Prospect Ratio:
You'd be hard-pressed to find a newspaper ad that will generate prospects at a cost of only $1.60 each. Now you just take these 167 prospects, get them into an effective drip email campaign, and do some business.
Though all the Old Marketing Venues Still Work, Look Carefully at PPC:
If your site is properly designed and set up to capture prospect information and work them through drip email, PPC can be extremely effective and not that expensive.