SEO Quotes for sites that have not been built

This week I’ve found myself in the situation a couple of times where I’m quoting for SEO work on sites that have not been built yet. It’s not a massive problem, but it is odd to not know what you’re getting yourself into. I can understand it from the company’s point of view – they just want a quote so they can budget everything for the new site. All that I’m often shown is the old site and I’m told it’s going to be rebuilt.
I approach these kind of quotes warily. You really don’t want to undersell yourself and end up optimising hundreds of pages of content for a few hundred quid. So in the end I quote a reasonable amount with a few provisos attached. For a certain amount I will look at and advise on this many pages. It’s all you really can do, and when a project goes over budget at least you have that to refer back to.