I treat an SEO like I'd treat a normal PPC campaign, I allocate a test budget like $1,000 say. Forecast what my average ROI would be for a 12 month period and if it's acceptable within my own standards I scale up like crazy, I don't even have a budget like I would spend $30K a month if it'd bring in $100K a month.
It's all about working out what your eCPC is and what your EPC is and if you're making $1 for every 30 cents you're spending then, get scaling that shit as big as it'll go!
Keep doing that until it becomes unprofitable, and then drop it like a rock.
Wash, rinse, repeat.