My background before starting Yourdoormat.com in 2000 was in advertising sales for Newscorp. and Time/Warner . CPM's were most important to clients. But you have to be in right medium and you have to be in a medium that enables you to create a reasonable amount of awareness (i.e. there must be a reasonable commitment from you).

Here is a quick way to look at CPM costs and we used to do this all the time - at least in the newspaper industry:

CPM is the cost of an advertisement divided by the circulation x 1000. In the end, CPM stands for cost per medium but we always talked about it as cost per thousand - or the cost for 1000 people to see your ad. You need to know how many people will be seeing your message (i.e. the circulation of a given audience) and the price for that given ad. Is your chosen medium audited properly? In the publishing world, ABC (Audit Bureau of Circulations) was the bible. With good, substantiated numbers, you can then do the above math.  

Importantly for ROI, are you in the right place? Is this your target? Is it affordable for you for a medium/long term marketing effort? To just get in and give something a try 1x is not worth the effort or expense. Frequency is also very important but that is a whole other subject to touch on. 

 
I have a great respect for finding a niche and servicing it well- real estate notwithstanding. Marketing is but one challenge and it's really easy to spend a lot and get nothing.