Really your own choice. You could, for example, base it on an analysis of the yield (more revenue) versus the costs compared with otther ways of promoting your website.
A contract with a SEO-company where the fee is related to the grow in paying visitors (to make cheating impossible) would be a good idea. If the promise 50% growth, they get nothing if it's less then 40, the agreed tariff if it's 40-60 and a sizable bonus if it's more than 60%.
How much should I really invest in SEO?