I have been doing business online for over 15 years and specialize in SEO as well as business/marketing consulting. I am a entrepreneur who LOVES business.
It's actually very smart to do it the way you are doing it. We do this all the time and have tested it extensively. What you want to do is to see if your ad is getting enough "reach" at the 20 cent click level. Reach is the amount of times your ad is shown.
If there aren't many others bidding, you can bid pretty low and still get lots of reach. However, if your ad isn't getting shown much, you'll know that you should raise your ad price.
Make sure you also test using Optimized CPM versus paying for clicks as well.
Hope that helps.
You can absolutely dominate with this (assuming your web app is excellent) by doing Facebook Ads. I recommend right hand ads that are targeted to people interested in various SEO keywords, groups and pages.
We do this for clients all the time and by far, Facebook is the absolute best (and most affordable) way to market products and services like this right now. The targeting, especially using the Power Editor, is ridiculously good.
I'd recommend testing 50 different ads and 2 methods:
1. Landing page that is on your Facebook page as a tab
2. Landing page on your actual website.
This works really well right now, even at lower ad budgets.