It depends on what my budget was. If I was a giant corporation, a few thousand dollars a month isn't unreasonable. However, the majority of budgets i'm guessing are going to lie between 300-600 a month.
It depends upon the site and the keyword I target. Ranking high for a keyword with lot of buyers is not the same with ranking higher for a site and keyword full of freebie seekers. -Neo
its totally depends upon your business, your competitor. if you will have more profit from seo to your business then is not saying that spend money for seo but in another language we can say it is investment for business. first you have to decide how much you invest in beginning of seo. then after some months you will see result of seo to your business then decide you want invest more money for seo or less.