Assuming a rain fly for your pack, since it’s in the backpack forum.
How about going ultra cheap and just using a contractor bag thrown over the top of the pack and then cut out for your shoulder straps to go through? Or, if you’re looking for something lighter wt. you could just use a thinner mil. trash bag.
Track down jimmy tarps. Facebook, eBay, etc. they sell pack rain flies that are great. Silnylon. $20-30. I have one in brown and orange. They also have e good, well made fairly cheap tarps. I have a 6x9 from them that is my ”just in case“ tarp.
I picked up a clearance price McKinley bag rain cover for $5 last year, orange though which is probably why was so cheap.
3.8oz and covers my 4000 cuin bag fine.
So I would search some clearance price backs packing websites if you wanna save the cash.