What you can use is the Overture Bid Tool to find out how much certain keywords are going for...but don't expect to pay less than 10 cents per click. You can see what their bids using the following link:
http://uv.bidtool.overture.com/d/search/tools/bidtool/
Please note:
I know Yahoo has some changes in process, so I don't know how much longer you can view bids...
You can also try MSN. I have not used them yet, but I hear you can get bids for as low as 5 cents a click.
As far as the smaller networks are concerned (e.g. Miva, Enhance, and others), I have heard that they are plagued with Click fraud, which means that even thought you can get cheaper costs per click, your costs per conversion turn out to be higher at the end of the day.
To see what I mean, go to
http://StatCounter.com and get free tracking for your website. After you have advertised for a few hours on a smaller PPC network, compare the number of clicks to the number of unique visitors, and you might get an unpleasant surprise...
You can get lower costs on Google by increasing your relevancy. To do this, try and use the keywords you are bidding on in the various "tags" of your webpage and within your webpage itself - this can help lower your cost per click. You can learn more about this kind of stuff at Wealthy Affiliate.