Army cutdowns aren’t bad – I use them for commuting. Very cheap, very waterproof and good fit on the legs. Problem is fit on the waist – they’re overtrousers so there’s no tailoring and they slip down a lot when you move around the saddle on a MTB ride (at least for me – they just have a drawstring on the waist). I’ve been meaning to get a set of braces for them that might sort that issue out.
Altura attack 3/4s that zippy mentions were great for me but they didn’t last. Wore the aris out of them in a year and that’s without using them excessively at all. If there was a re-inforced backside version they would be perfect.
The robustness issue is the real challenge with kecks I think. A garment that is comfy, waterproof and can take a couple of years of hairy-ersed riding on wet British moorland is a tall order. Throw in not too expensive and it’s moon on a stick time.