Inexpensive window coverings, any suggestions?

Try static cling window film. It will help with insulation and filter the light.

Blackout curtains that are insulated. Overlap panels and wrap around on the sides rather than a straight rod.

We use the window kit every year on one window that gets very drafty. Check the seal and caulk it up as well if it leaks

I picked up curtains at Walmart for $10 a pair (so one window) that block light and help reduce drafts. They have more expensive panels that are thicker, but I've been impressed with the cheap ones I bought.

Home Depot has paper shades that tape to the inside of the window. I haven't tried them; but I considered them before installing plastic shades.

I use drop cloths as curtains all over my house! They're very budget friendly and so easy to make! They require no sewing. See them here: https://www.aratariathome.com/blog/diy-drop-cloth-curtains

Hang some cheap quilted blankets over plastic sheeting or bubble wrap

Check out Amazon. I just bought 2 sets of 2 panels 50x63 each--for 5.99 per set. They are nice. You can also buy liners to go behind your panels. I always check the clearance section at Lowes for curtains too! I have gotten some really nice curtains for just a little bit of money.