Decide how much you have to spend on decorating, and that is your budget. Drapes can be as inexpensive as a set of white sheets dyed to match the rest of the room, or as expensive as custom made drapes. If you can sew, or have access to someone who does, then your expense will be minimal. Remember that the sun will fade your drapes, so use lighter colors and the sun fading won't show as quickly. Heavier drapes can make a large room look smaller, and cut down on echos for a more intimate feel.
Drapes can hide a small window or minimize a large window.
The look you end up with is up to you. Furniture is the next biggest expense. Re-upholstery is not hard, but can be time consuming. Pick a color that suits your lifestyle. Slipcovers can be attractive and are usually less expensive than new furniture and re-upholstry.