If you aren’t a ‘bib guy’ (I’m not- they make my tummy hot), the cheapest option may be a pair of unpadded 3/4 length running tights. I’ve got a Champion-branded pair that I pull over whatever shorts are clean that cost under US$20. Comfy from 5-15 degrees and can be tossed in a pack if it gets any warmer. Keeps the hips warm too, something that can’t be said of knee warmers. Be warned: you may need to brave the ladies’ section of your local big box store and make an educated guess on sizing.
Want to spoil yourself? Ibex’s dense wool El Fitos with the pad cut out do much the same but are good down to about freezing. £110 without bib straps, £130 with. Amazing and – as you’ll be wearing clean shorts underneath – good for several rides without washing.