There are many false prophets offering miracle SEO solutions. Often they provide nothing more than a load of poor quality links by spamming online directories and forums. Often this is looked on unfavourably by Google and it certainly does nothing for your business reputation to have some clown posting on your behalf in trade related forums.
In my opinion web developers should take a broader view on SEO and it should be seen as one strand of a complete digital marketing strategy -
1) in terms of the site itself the operative word is 'optimisation' no one can promise 1000's of hits or keyword world domination but an SEO expert worth their salt should ensure that your site will be as effective as possible based on industry assumptions around Google's algorithms and on the keyword research they've conducted for your business.
2) they should provide regular top draw new content for your site - e.g. blog management - I'd want to know their credentials on this, see previous work and agree topics for pieces in advance to ensure direct input and a degree of quality assurance.
Original content should be about your business, market your services, build your reputation - of course it needs to be keyword rich but SEO benefits shouldn't come at the expense of your credibility or undermine your product.
3) I'd expect under this wider strategy to manage relevant social networking and promote your business via Facebook, Twitter and industry relevant blogs and forums. Again this should contribute, raise profile and network.
4) They should report regularly via analytics and adapt the strategy (in conjunction with the client) as trends develop.
5) this is a long-game, there is no quick fix.
I wouldn't expect to get this for £20 a month or whatever these cowboys are charging but it's a damn sight cheaper than hiring a full time marketing manager.
Really I want to stress that there are options out there that are provided by design and marketing experts who can really enhance someone's online presence but that it's important to view SEO as only part of a wider strategy.