Make cute Valances. There are tons of them to consider. Look on Pintress or Google simple window valances.
What do you have to start with? None of the window treatments in my home are expensive. Do you have a rod? I prefer a dual rod as it gets really hot where I live in summer and need to close up during the heat of the day. Picked up the rod on sale a Bed Bath and Beyond on clearance. Used sheer curtains underneath that were $5 per panel at Big Lots and curtains I found at Target at $25 per panel, but waited to purchase when they ha a 25% off sale.
Or you can do a curtain rod with curtain clips and a dropcloth.
You can make easy and cute window valances: https://momhomeguide.com/hanging-window-valences-april-pinterest-challenge/
Check out JCPenney's website. I was able to purchase some room darkening curtains for $7/panel which is cheaper than I could make them.