I did this for the last several months….
I had two PR0 sites… One was 4 months older than the other. On the oldest site I bought a couple of PR7/8 site links.
That site with the purchased links became a PR3. The other which I gathered my own links from quality sites is PR4.
Don’t know if that was the determining factor… but the real point is that it WASN’T a factor 
kw