Personally I've found the cheapest windproofing for your run fence to be 6 mil translucent plastic, like they sell for vapor barrier purposes. If you have only a small run and do not need the giant rolls they sell at Home Depot you should be able to find someone with leftovers from some project, or just find any other stout plastic to use. Stretch tight, and staplegun on through duct tape, or attach with battens. Use LOTS of staples or screws. If you find the plastic becoming baggy and flapping in the wind, install it tighter next year
and for this year get some string or old baler twine or whatever, and crisscross that back and forth to bunge the plastic down tight; or tack some old scrap molding or used 2x4s over it in the same way, to discourage flapping.
But before deciding to do this make sure your run fencing will not blow over due to the resultant wind load.
In a similar but even more serious vein, DO NOT use plastic (or anything else solid) on the top of your run unless you are REALLY darn sure the supporting structure of your run is engineered to withstand the resulting snowload which can be quite considerable even from not very much snow. It can be quite a pain in the neck to try to repair a collapsed/ruined run, especially in the middle of winter.
My preference is to leave one side, or half of one long side, of the run completely un-plasticked, for ventilation, rather than a gap all the way along. If you WERE leaving a gap all the way along, though, in principle I would suggest it be at the top rather than the bottom, so you do not have cold winds blowing right at the chickens which would discourage them from using the run on nasty days.
JME, good luck, have fun,
Pat