$450 for a pair of pants? Not in this lifetime! Even if I won some huge lottery, I would pay off my sons' houses first.

Look at these sale pages from some USA retailers:

A sponsor here: http://www.backcountrygear.com/asearch?q=mens+softshell+pants




Or this Canadian one:

I don't know the international shipping policies for the American companies, and I didn't look up the details for any of these, just sources for softshell pants. There are others, of course, but I have purchased from all of the above over the course of many years and think well of them.

If you are willing and able to make alterations to meet your specs, go for it. IMHO, you needn't be overly concerned about sweat build up if you add nylon over the knees and seat while the rest of the garment breathes. Sweat should still be able to wick to where it can escape, for the most part. There are some WPB fabrics that should be lighter and more abrasion-resistant than the main softshell fabric, it you are determined that the entire garment be breathable, but they may be less durable than, say, cordura.