Quantifying the value of a link?

Hi guys, have about a year and a half experience in SEO and would consider myself to have a good idea, but something I cannot get my head around is quantifying the value of particular links.

I always think everything is either way way too expensive or seems like a really good deal. I know all about checking link metrics, PA, PR etc etc. Check the backlink profiles and have all the information in an excel spreadsheet but can't seem to create a value number out of that information?

If I can give you 2 examples you can maybe tell me how much you would pay for a link and see what people say.
(This is more advertising based than just purchasing links)

Site 1.) A charity website that is PR7, MozPA 72, Moztrust 6,27, linking c blocks 375 about 1200 linking root domains.
With also around 250,000 UV a month.

Site 2.) A niche blog that is PR4, MozPA 46, Moztrust 5.4, linking c blocks 43, about 87 linking root domains.
With also around 250 UV a month.

Now any idiot knows site 1 is a better link BUT how much better? In terms of money and what people would pay for a link (or advert) on their site.

Site 1 - 1 year of advertising (links on homepage) £2,000.
Site 2 - 1 year of advertising (links on homepage) £50.

Any advice, thoughts welcomed.
Thanks
Tom.