With a 300mm zoom compared to a 300mm prime, IN THEORY the manufacturers would make their degrees/field of view the same (somewhere in the vicinity of 8 degrees - give or take about .5 degrees) so there should be no visible difference between a cheap one and an expensive one. Feel free to get a less expensive one which will be lighter - but like you said, not as good construction and not as wide an aperture.  Always a tradeoff - funfunfunf!

If you're asking about the "magnification" factor, with the D100 it will be x1.5 meaning you will be using the equivelant of a 450mm lens.
Hope that helps-
-Lew ;-)