I'd also second IKEA. If one was closer to me, I could probably furnish my entire home with their stuff! I tend to surf the clearance racks at our local Lowe's stores--can find some really good deals on items like mirrors, light fixtures, etc. Craigslist or Facebook swap pages are good for used items (some in excellent condition, others that might need a little TLC). Although, expecting to pay $100 per light fixture isn't really "budget" to me... my goal would be closer to $25-$40 each! If you need help coming up with ideas for arrangements & decorations, visit all of your local furniture stores! They do a pretty good job of laying out all the pieces in specific rooms.
I think you might really want to consider if you're just trying to furnish the place for cheap (and temporary), you can find some really good deals. If you're truly looking for good long-term items, you will have to increase the budget. It just depends on what your goals are!