The question should be how much is it worth to you. What ROI do you expect out of it?
IMHO, I wouldn't do this just for the sake of building a strong link. Something like that should bring me money in the short term, otherwise I can invest 1000 USD in many other ways to just push my rankings.
Sure it is also good for showing off and build some credibility, but again, what is the return and how long will it take?
A link like that is not just a link, you need to think how it fits in your business plan.
Maybe you can let us know how much they ask you in the end...