Not cheap and obviously their thing is either your cup of tea or not but…… I just picked up a bargain nearly half price rapha rain jacket on ebay, took a while to find and win one.
Its the best road waterproof i’ve tried. super comfy, packs into a pocket, slightly stretchy, great cut and REALLY waterproof. Wouldnt have stretched to full price but it’s well worth £100. I had been using a nice montane pertex one but it’s much better IMO
Worth remembering that it’s gonna last longer than your mtb ones since it wont be arguing with rocks or brambles at all and that being wet and cold is more miserable on a road bike than an mtb.
I think dhb (wiggle) do a fairly cheap good waterproof