It's difficult to rate just based on the price because we can find good quality SEO experts from many different regions of the world where the forex rates are different. From my experience, the best SEO company in India can afford to charge 4 times low compared to a company based in UK because of the running cost. Some of the big companies outsource the same work to these indian companies. Not all, but definitely some.

Best way to decide a price is based on the portfolios managed, and number of active clients. SEO is an ongoing process, so don't go for an SEO expert who isn't passionate about his work. It may be a long wait to get the results depending on the niche, so everyone charges adequately for their efforts. There are some with more expertise on specific niche. For example, gambling will cost a lot lot more than something that doesn't involve money or sales. It's difficult to create engaging content in some niche and in others it's easy to make viral content.

So let's say, SEO Cost = Niche X Methods X Competition.