This info-graphic (albeit in dollars) by the monster internet experts over at MOZ gives you some idea of what people are paying for their Search Engine Optimisation services across the world (see below graphic for a UK relevant summary)
Infographic by SEOmoz & AYTM Market Research
In a nutshell
Hourly SEO costs vary across countries, but £50-£130+ per hour is the most common
Project Pricing is also popular and is generally between around £660-£4950*
Monthly retainer is still the most widely popular and most commonly used by SEO agencies small and large. The two most common pricing structures were between approx £165 per month and £329 and £1648-£3300 month.
Start up companies may charge you less for mutual gain. You get expert services cheaper in exchange for recommendations and good reviews. Of course, the fees also depend on what services each company offers and which services you choose.
For most businesses today, SEO is the highest ROI marketing effort. The benefits it provides far exceed the value of other marketing approaches – direct mailing, broadcast advertising, online ads, etc.
Businesses no longer decide whether they need SEO services. Instead, they decide how much they’re going to spend. A wise choice of SEO company will lead to incredible amounts of revenue. You can decide how much that’s worth to you.
*These findings are four years old and prices may have increased a little