Couldn't this be done with a property on the page that you set from 1.0 to 0.1 and then use that for sorting your result? Or use it in conjunction with the Boosting?

Yes, it could, but I was hoping there already was a built-in feature for this? Seems like a good function for a search index

Well the search will calculate a simular number, but as this number is calculated for each query and thus dependent on what you searched for the number will not be set on the page. 

So I guess you could dicuss if it's good or not to have it.

For me the the Priority value that google Sitemap uses is just for you as a owner to determent what page within your website is the most important. For most website I'm hoping this would be the start page (or in the case of a ecommerce the purchase page ;) ). But I don't see the winning in setting a hard values like that. As it doesn't take into acount what my users are searching for. 

Well the client wants it regardless the level of sanity, and the Boost thing you can do in the search would be as good (I mean bad) as this I guess.