I think I may be getting 'off topic' here as the initial question was alluding to the cost of SEO and thanks for the comments regarding why people don't include it in business plans however, I think I need to clarify some basic points regarding employing an SEO company and this is where I totally agree with when he says if someone says they can guarantee results then run a mile! However, the key to making SEO effective, trackable and have the ability to show the return on your high investment then your SEO company should be discussing with you in the very first instance what it is you as the customer wants to achieve. Now, this is where good communication comes into play. If someone says to us we want to be number 1 on Google for 'bespoke computer software' and we have a budget of £90 per month then obviously we would have to say that it is completely unachievable. Similarly, if someone wanted to be number 1 on Google for that same phrase, had a budget of £1K per month but the website would not allow a decent amount of conversions then again we would have to say that it is unlikely that their enquiries or sales are going to increase because of their website.
People are under the misconception that SEO is stand alone, this is definitely not the case. In an ideal world when you are looking to get a new website built you should get your SEO, designer and developer in the same room together. There will always be a compromise between SEO, design and development, but people must be aware that you tie the hands of the SEO company if you want a flash based site and no easy way for a customer getting in touch with you. This is when people become unhappy with SEO providers because they feel they have spent a lot of money and havent seen any return on the investment, a proper SEO company will go through all these constraints with you and give you an honest opinion on what will and wont work for you. If your budget is only £90 per month for example then it could be a case of making sure the meta statements are correct, that each page has some on page opimisation and that you should actually then concentrate on some directory link building - as long as you as the customer is aware that £90 is going to buy basic services then there are no unreal expectations.
With regards the building a fence analogy, I totally understand that, however if you stick with that example and a gardner comes round and you say here's £50 build me a fence around the garden and he gives you a 1ft high piece of plastic fencing around the whole garden, you have your fence but it's useless. What he should have done is say to you at the time your £50 is only going to buy you a rubbish low plastic fence are you sure you wish to go ahead with this? Or he would have asked you what you wanted the fence to do, keep the dog in or the neighbours out and actually you need proper wood panels and that will cost you £500 - you get my drift?
We offer start up packages for set number of pages websites, however, at the time we make it VERY clear that the end result will be that their website is friendly to Google and that there is probably no way that their website is going to be number 1 on Google any time soon. BioOutsource made a good point here that SEO might not always be appropriate, what we would probably say in this instance is that make sure your website looks good from a code point of view for Google and if you budget is not going to stretch to ongoing work then look at raising your brand awareness yourself through business directory submission, Facebook, Twitter and the plethora of other avaialble and free social media channels - as long as you have the time to do it!
The end result of all of this is, if you want a proper service you would pay a professional to do it for you if your budget allows.