Ask HN: How are you measuring keyword performance?

I started some basic analytics and it quickly became clear that the lifetime value of customers who come in on keywords is substantially different than the LTV of customers who come in on other keywords.

I'm not a big enough spender to simply call up Marin and ask them to handle things for me.

As such, I'm wondering what I should do. And what better proxy than to ask what you guys actually do?