I'm not sure how effective proofing them will be. You could look into one of the proofing sprays you can get in outdoor shops. However, its a major problem with cheap boarding pants. You should be able to get a spray for less than a tenner.
But your main problem will be when you sit down, either on the slope or on a lift up (if its snowing ) as your body heat melts the snow and (especially cheap) pants then soak it up. So I don't think merino would be a good fix as you'd still have a wet ass.
I'd recommend a pair of impact shorts. I've had a pair of these now for over 7 years.
As well as the protection they offer, they also lift you off the snow a bit, and also keep your cold/wet pants off your skin. Also as another bonus, you can remove the coccyx pad and it's ideal for MTBing as well :).
Ideally you should invest in a better pair of boarding pants, Sport pursuit (yeah that website) has some cracking bargains as does tk maxx. But even with quality pants, I wouldn't be without my impact shorts