Kind of like jogging pants or track-suit type of pants, you can usually get something at KMart that is all nylon, they often have a mesh inner, and zip legs. Check labels for 100% synthetics. Over your longies they will be quite warm.
Other good stores to check for cheap are Ross and Marshalls.
I heard Mervyn's is going out of business, if you have one.
Try to blow through the fabric, or suck air through it to see how wind resistant it will be.
Light colors can help keep from overheating if you have to wear them in the sun or transition alot.
They can blow around too much in high winds, trip hazard if too loose at the boots. For climbing, the legs may be too big around. They can bind on your longies at the knee and cause resistance when climbing. May see some with articulated knee. Some have smoother linings at least near the knees to facilitate slippage. Some have zip pockets. No pockets may be preferable to open hand pockets, or can sew them up.
Can add a tie point on the cuff to tie them down around the boot as half-as gaiters. They often come with a useless waist cord and stripes which can be removed.
I have some powerstretch fleece loose (manly!) tights that have an ultra wide range of temps. With some altered nylon Wilson Golf microfiber pants, 20-30 degrees is great if I can move around. If belaying someone all day down in a gorge, maybe a couple sleeping bags on the crash pad? You need thickness of insulation if stationary.