I need to get some riding pants...

My first pair of pants were the Tourmaster Flex something or other, and I was really happy with them. I could only get a size small so I didn't wear anything under them (and I still had to suck in my gut) but they worked pretty well for a $100 pair of pants. The thing I loved about them was the flexibility. They had an internal waterproof layer you could take out and on the outside, you could actually zip off the top layer leaving a mesh layer beneath. Pretty flexible. Not sure how waterproof they actually were as on my scoot my legs are really protected from the elements.

If it's just a waterproof layer you are looking to add (assuming you have protection already covered) then I can't recommend enough getting a separate waterproof layer. ie. not motorcycle pants, just rain gear.
I fought going down this road for a long time but wondered why once I did. Admittedly I lucked out and used my waterproof pants from my skiing outfit (which had seen about 2 uses over 3 years) and my waterproof jacket from my cycling days. So for me the price was right! If you get decent rain pants that can breath then you should be set. I think my pair is Marmott something. Last I checked I think they retailed around US$120. They are designed for hikers so they have a toughened layer around the ankles to prevent tearing. Don't think this would offer protection from a hot exhaust though but it's nice to have.