The cost of the SEO service will depend largely on the system chosen, since PPC (pay per click) is much more expensive than SEO.
On the other hand, there are many companies that offer economic positioning services, but they are nothing more than a deception, since a professional in positioning knows perfectly well that positioning a site takes time, work and effort, so it cannot be so economical and it is well said that many times cheap is expensive.
That is why rather than seeing the positioning service (SEO) as an expense, we should see it as an investment because once we start the positioning correctly, we will see the results in the increase of web traffic and this in turn in sales.
In conclusion, SEO services should not be framed in economic or expensive, as this will depend on how much we want the success of our company.