Good SEO link building services can not get for cheap price. If any company is offering very low price means they are cutting your pocket by do "black hat SEO" which will hurt website ranking, decrease in organic traffics and can be reason also of Google penalty.
SEO cost vary according to niche market competition and number of keywords. Good SEO company like SEO Manual Submission charge a bit higher but promise for result oriented services and use there long experiences to give you desired goal.