My sister did her daughter's room in a princess theme about a year ago. Since then I've seen all sorts of great inexpensive items. Hard for me to put my finger on all the specifics, but if you keep your eyes open there is TONS of princess stuff to be had at low prices.
Home Depot has offical "Disney" paint to match the Disney princess items.
The Party store had lots of cute stuff for parties, some of the items - like tiaras- could be good for longer term decorating.
For a party, I took a huge box and painted it to look like a castle. It came out very cute. You could also consider using a slightly sturdier material for a more solid structure. The acylic paint (I also watered it down) was very inexpensive at the craft store. A year later the box still looks decent! They also have this really cool speckled spray paint now that makes a surface look like stone. You just paint the surface gray, and they spray this stuff on over it. It even gives the surface a little texture. I used it on some outdoor pots a few years ago, and they still look great.
I've seen the netting for over a bed a number of places. I saw a net, specifically for over a girl's bed somewhere very inexpensive - maybe Target or Kmart? Target also has cute room decor, maybe not princess specifically, but they have things like door beads and purple hampers, pink lamps. Check back periodically as they restock every season. They had pink "princess" items for dogs yeterday. (placemats for example) They also had kids' tables for under $8. (see the summer picnic item area)
You could get tulle (veil material) at a craft store like Michael's (any place that sells wedding decoration/favor stuff). They often have various pastel colors. You could make your own bed netting, or curtains (just be certain that the material is flame proof!)
Watch Disney Direct for sales. Also, if you have one nearby, the
Disney Store has sales all the time. After last Halloween, we got a costume for this year that was very inexpensive. A week later, and all the left over accessories were very inexpensive - like $2 each! - for shoes, wigs, wands, etc. You could have wall pegs for hanging various costumes, and a framed mirror.
Static stickers are inexpensive, and they stuck to a wooden dresser very well. They can be peeled off easily without leaving any residue. I'm not exactly sure where to find Princess ones, but they are around. They would also work well on a window. I've also seen "jewel" static stickers that could be used to create a very cool stained glass effect on a window. Check a stationary or wrapping paper dept. I've also seen them on checkout isles.
Our local mall has a store called "Club Libby Lu," that specializes in all sorts of tween girl stuff. They do special up-do's with glitter and all that jazz (great for girls' b-day parties), but they also sell room decor. I can't say they are particularly inexpensive, but they have some smaller items.
You might be able to find appropriate movie posters somewhere.
Any where that has a good selection of picture framing (Michael's, for example) will have special 3 dimensional frames that you can put small items into. I forget what they are called, but they look really nice, and the items stay dust free (behind a glass or acrylic cover). You might even be able to find one shaped like the magic rose bell jar from Beauty and the Beast.
One book in particular that has gorgeous illustrations without words on one side is the "Disney Princess Play-a-Sound" you can find these very cheap on ebay. The pages are also very large so it will look like professional art when framed. 
Good Luck!!
