Note that breathable waterproofs shells are best for rain riding. In dry or damp conditions, they can cause condensation. A windproof shell and a separate waterproof shell are a good combo and also work together to add warmth on unusually cold nights.
I dont think you need your rain pants to be as high grade as your jacket.
You may be able to get a summer-weight, ultralight hiking waterproof in end-of-season sales. The only problem with them is sometimes dark colour, lack of reflective bits and permanent flappy hood with no roll-up into the collar.