Take the amount of revenue you want to generate from your website and calculate 8%. So for example this website has a target of generating £250,000 worth of new sales (it is well on its way). 8% is £20,000.
Now whereas traditionally your would have added 3% +/- depending on whether your company was a brand leader or a newbie now you can add +/- 3% for whether or not your website is well established (read old domain). However you can also add +/- 3% depending on the size and competitiveness of your keyword.
So in reality here at benedict.co.uk we have an established website in a highly competitive market (SEO) so we can add 3% taking our budgeted spend to 11% or £27,500 per annum, which is not far off our first cost.