Hi guys,
I know that a link from a relevant, high PR site could be really helpful, but what if the page which links to your site doesn't have a PR? Let's say; there is a PR5 site and it links to your site in an inner page which doesn't have a PR. How will Google consider this link? Will it be considered a high PR link or an ordinary link? I'd like to get your opinions on this. Thanks in advance.

Two things:

1)
Google visible PR is delayed many months

2)
It's the PR of the linking page that counts

(however, besides PR, there might also be some "trust" score that will matter when it comes to ranking in search results. Can you get an .edu link from some university blog or whatever then...)

Did you use a SearchStatus toolbar to determine that site is NoFollow or DoFollow link? In you scenario if that PR5 site is NoFollow it can't pass the link juice to your NoFollow link so your PR's web page is still N/A. If that website website is DoFollow it can pass the link juice to your link.

You ca use A1 Website Analyzer to try determine how much of the internal website link juice flows to the given page you can get a link on. (It respects nofollow, noindex etc. in it's calculations)

Use of the most appropriate keywords in ones website or content can also be another way of getting back links from high PR website.

A link from a great website always helps and is of a great value. But when you consider PageRank, it is only the page from which you have the link from is considered.

definitely your inner page will get the link juice if and only if it is dofollow link..types of link whether do follow or no follow can be through firebug too.

As far as you are getting a link from relevant resource its ok, just make sure that the site or page from where you are getting link from have quality off page done on it in order to keep yourself away from bad neighborhood.

And as far as PR is concern you don't need to worry about it as that particular page from where you are getting link is already been linked from other channels or through its site itself, and as PR flow from link to link through incoming links and internal links m sure that page must be getting link juice from other pages that are point to it.

I think first of all there is need to know that this link is do-follow or no-follow and if it is do-follow then it obviously give you link juice and advantage.

do-follow and no-follow discussion is a huge topic which is still unrevealed. Most of the SEO's believe that even no-follow passes link juice not as much as do-follow does, but there are still no solid proves by google or any other search engineer. If there are? i would definitely like  to learn about it.

Did you use a SearchStatus toolbar to determine that site is NoFollow or DoFollow link? In you scenario if that PR5 site is NoFollow it can't pass the link juice to your NoFollow link so your PR's web page is still N/A. If that website website is DoFollow it can pass the link juice to your link.