How to create an inexpensive window seat under bedroom window?

https://www.pinterest.com/usofeh/window-seat-ideas/

Here's an idea, and you will see there are many more videos of doing this.

https://www.youtube.com/watch?v=1esf7PVSpk8

I have a cedar chest that fits perfectly under my bedroom window and I use an old trunk in my living room. By adding a decorative cushion or pillows it serves the purpose very well.

I've built an elongated cube out of 2x construction lumber, attached it to the wall, and covered with plywood. A lot depends on what else you have going on around it. Last one I built was between two closets so it fit right in. Covered it with a matching carpeting but fabric would be a nice look. Had someone make cushions that fit the space and hinged the plywood top so we had a place to store the air conditioner during the off-season.

Two strong boxes and a scaffold board or an old Blanket box

Lots of photos here. Click on a photo and go to the site.

https://www.google.com/search?q=build+a+window+seat&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiq3b-82OvWAhWorVQKHe8uARUQ_AUICygC&biw=1016&bih=642

Inexpensively? Find an old wooden hope chest or toy box or storage box. Paint if desired. Add a cushioned top and /or a collection of throw pillows.

Check Craiglist for folks giving away or cheaply selling cabinets or bookcases and just repurposethem.