Most SEO companies I've come across charge £200 a month minimum - and if I'm honest even the "reputable" ones have plenty of bad reviews too - - kinda like most canvassers I guess!
I'd be very careful though. A lot of SEO companies use link-building techniques and backlink spamming to give your site a boost - a technique that gives great results short term (usually long enough for them to tie you into a 12 month contract) but means bad things long term,... in extreme cases it can even get your domain blacklisted from some search engines.
My advise would be to concentrate on content creation instead. Upload weekly blogs to your website, add pages, photos, even make a few videos,... fill your domain with high quality, unique, relevant & interesting content. Content is king, its risk frees (So long as you don't copy from other people!) and it will always help you climb Google rankings - although it is a bit slower to get going that traditional SEO.