It's more important to spend money on a good rear rack where the weight is carried.

The problem you'll find with cheaper racks on the rear is that while they may well be strong enough to carry the load they may not be stiff enough to do so without sway. That's why the Tubus racks are so solid. They are extremely rigid which prevents the bike from shimmying and swaying at high speed which can be a problem if you have a lighter more flexible rack.