Ask HN: What are average conversion rates?

I have a couple of ideas, (OK, more than a couple) for projects to work on. Some are physical products, some are web apps, there's even a game. I need to pick one, and the logical way to do it would be, pick the one that pencils out as highest annual profits. Obviously any amount of number crunching you do up front is wrong, but you need to start somewhere.

I personally believe, after having devoted a year of my life to a site that almost nobody ever came to, that any plan that involves "getting bloggers to blog about it", "making it go viral", or "just build it, and they will come", is not a plan that I want to be a part of. Therefore, I am evaluating my ideas based on one metric. How much does it cost to get the traffic and is that number, given some conversion rate and lifetime customer value, less than what that traffic will make for me.

Yes, I know that all of these numbers are crap, but again, you have to start somewhere.

Google has a great tool to tell you how much traffic you might get, at a given maximum bid, so I'm using that to tell me my cost of traffic.

My initial attempts to evaluate my ideas assumed various cost of goods and sales prices for the physical products. For the web apps, I assume a lifetime customer value of one year of subscription, conservatively assuming they'll churn out after that time.

So, it all comes down to what I think I can sell for, and of course, conversion rate.

My question is, please help me get a feel for what a reasonable conversion rate would be. After googling, I see things like physical products have much, much higher conversion rates, sometimes approaching 8%. These numbers drastically push me towards the physical products, which were getting their butt kicked when I was assuming 0.5% or 1% conversion. However, they seem too good to be true, and that data is over 5 years old.

Does anybody have any good data or even anecdotes on conversion rates for various classes of products, physical, web app, or game?