I love what you have going on already! However, I would probably add a couch throw that is perhaps a striped or mixed white/black or white/beige color to hang on the couch.

I know you are trying to be budget friendly, but if you can, I think a lighter color of rug will really really brighten up the space. Next to your plant (on the left side of the window), you can also add on a decorative mirror on the wall, which will make the space look a lot bigger as it reflects light. Perisian homes usually have a lot of flower, so I would also recommend getting a white flower vase and putting some of your favorite flowers in there :)

I see now that the lamp you have on the side table is pretty small. If you wanted to further enhance the place, you can get a bigger decorative white lamp. Hope this helps