Hmm. I’d recommend hitting your local thrift store in your under layers (I’m presuming you’re throwing these on top of something) and try some things on. I find I usually rotate between slush pants (bought from costco, very lightweight, light fleece liner on the inside) or snow pants (again from costco, relatively light weight and stretchy so easy on). I do like to buy these a size up for layering underneath.
I wonder if a light weight unlined overall from Carhart or the like could work - we have these at our local hardware store. But honestly, the cheapest would probably be a pair of baggy and lightweight jeans. Quite possibly from the men’s section, depending on your preference and body type. So again, the thrift store could ver well be your easiest (and cheapest!) solution - and you’re saving the planet at the same time.