I would never pay $100 for one generic backlink just because the site is high ranked.
But I would DEFINITELY pay $100 for a banner/link that is on a site with the same content as mine. A few months ago I started a site and payed $25 for a 1 month banner ad that made me more than $10k that month. Use your head. Pick a product, make a site, do the best SEO as you can, but if you can't beat the competition, then join them. Find a cool review site with good traffic and use the Contact page to ask for advertising costs. I was lucky enough to find a guy that just wanted $25 a month, but if you're selling something that's $50 and the site's admin wants $100 a month then you better make sure the site is good enough to get you atleast 2 sales to pay for advertising costs, which should be fairly easy.