there is no easy answer. depends on where it's placed, how prominent it is, and how much traffic/benifit they are going to get out of it. i can't see they'll get much, if any, if your site is brand new with a PR of one. maybe they are thinking long term benifit.
as londrum answered pretty much everything. personally i wouldnt buy a text link from a pr1 unless you have a lot of traffic. i stick to selling banners and page advertising . it is a lot easier to sell and people dont tend to care about pr, only visitors.
If you don't sell links excessively, but present your "sponsors" properly with picture and textlink ("advertorial), I think that links from the homepage should go for a Euro per 100 visitors - eg. if your homepage receives 500 unique visitors a day, I would charge 5 Euros for the link.
But this, of course, is only a rule of the thumb - you see links for sale for much less and much more; on average, the rate I gave is rather high and you won't sell many links with it.
[edited by: encyclo at 2:01 pm (utc) on July 23, 2008]