A business (or anything for that matter) is only worth what someone else will pay.
I'm involved with a number of ventures, and while there's an inherent value of each business, the real value is determined by the person who wants to spend the money.
With regards to PageRank increases - that's a tricky area as you don't actually control that factor. In a moments notice, you can have that ranking yanked away from you by Google - so even projecting for the future can be tough. I watch Flippa from time to time also and I'm amazed at some of the figures people throw around, but if that's what they want to spend, that's what they spend.