So you put a page with a description of the idea, put up some ads using AdWords and you had say 10 clicks with a CTR of 0.45%. Of the 10 clicks, 1 person signed up for a newsletter.
Is this a good result? Or is this bad?
The person signing up obviously is making the decision solely on 1 page. If I were signing up, I'd look at the website. How much activity is on it? Are there any blogs or videos? Should I even plug in my email even though I know I'm not paying anything?
Now let's ratchet up a little. What if I was charging $99 for something? What metrics (clicks, CTR, etc.) should I be focusing on?
Obviously, all this will depend on the type of product/service being marketed, the competition and a whole bunch of other factors.
How should I go about thinking about this lean startup methodology?
I would caution that the conclusions you draw from this might not match actual performance when you start selling your product. It's been my experience that people will gladly sign up for "free stuff", including worthless news letters and such, and will investigate things they've never seen before out of simple curiosity, but when you put a price tag on it... well, people get weird when money's involved :^).
With that said, I think the plan has some merit IF it's a completely new product or if you utilized those click throughs to collect non-passive data. For instance, rather than just offering a free newsletter, put up a "help us make a great product" page and ask for information about what they are hoping to see. This would let you know that it wasn't just a passive "what's this" sort of click, AND it would get you useful, meaningful market data on your new product, or a marketing angle for that product to make a bigger splash when you do release.
Hope that's helpful.