what is a proper price to pay for a link with, say, PR5? and is it better to buy one or two high PR links as opposed to several low PR links?
You'll find prices vary quite a bit if you look at it just from a PageRank perspective; link location, traffic, demand, topic, etc., all influence the asking price. A home page link will likely cost more than an interior page with the same PR, a link above the fold would be preferred by some over footer links and so on. You should be able to get lower PR links from link exchanges, trades, directories, etc. so I would spend the money on a few better quality links as opposed to a bunch of low value links. I wouldn't rush out and buy a PR8 link for a site that had few other links, especially if they are lower quality (PR) links. Try to achieve some type of balance/ratio that might lok a bit more natural. Remember, it's buyer beware - faked PagRank, "nofollow" attributes, robots meta tag and .txt file exclusions, javascripted links - the list goes on. Be sure to examine the link so that you know what you're getting. And I wouldn't buy a link on a site whose domain name registration is set to expire before my term was up - leaves me with an uncomfortable feeling. For a PR5 link I'd expect to pay between $5 and $15 a month just for an average. You might get term or quantity discounts. Never hurts to ask. I'm sure you'll come across some for more but few for less. You'll need to determine the value of the link to you when deciding if you might buy. Another option is link brokers, but realize that there's a split on the monthly fee and you may pay a premium over buying direct.
Also factor in how many links are already on the page. If you can find a solid PR site that isn't linking to anyone yet, you probably can get a better deal because the webmaster is not thinking of selling links and will be thrilled to monetize the site somehow, as opposed to a site that already has determined a price and doesn't need your business.
A quick way to see what links are on a page is (if you're using FireFox) right click, View Page Info, click the Links tab, and voila!