If you are in the market for SEO services, there are certains things to know.
SEO services.
When looking for the services of a SEO firm/specialist, it is better to seek out the service, rather than respond to cold-calls or unsolicited proposals. This educates you on what is available. It allows you to get a specific quote on your specific site, for your specific markets.
Any SEO firm/specialist generates "generic traffic". Dong this while targetting specific search engines only takes modest skills. However, in most instances, this does not help you at all.
Determining a price.
A flyer, quoting price per page, without knowing your industry is worthless. A quote requires knowing...
industry competition,
searching habits of potential customers, and
the current (or future) content of the website and its products or services.
Otherwise, the effect of using SEO services is doomed to fail.
SEO "costs" and "time" are inseparable. The faster you want, or need, results the greater the cost. In many cases, if not done properly, you see no results for a long period of time.
Spreading the net.
As the number of web pages optimized increases, the more targeted keywords (niche markets) you reach. In the hands of a less capable SEO company (generally those mass marketing), they optimize Meta Tags, only.
This is the bulk of their skills and the "optimization" of your main page. They then duplicate this across all other pages. This type of service does nothing at all toward getting traffic to a website. If you are paying per page optimized, you are wasting money on each page.
Conclusion.
The ideal situation is to learn SEO, time permitting. It helps to understand the value SEO services provide.
Mass-marketed SEO services, particularly ones generalizing on deliverables, are best to be avoided.