settle this for me: how much does search engine optimisation cost?

honestly the range is so wide it's almost useless as a number. a freelancer for a small local site might charge a few hundred a month, an agency for a bigger site several thousand, and a one-off audit is usually a flat fee. for the client, the better question is what they get each month. if a quote can't tell you what work happens in month two, that's the problem, not the price.