The most reliable large volume filler is your own fatty tissue. Areas such as the buttock and hip can require large volumes of a substance to adequately fill the area and produce a cosmetic change (500-1000cc per area).
Standard fillers such as hyaluronic acid (Restylane or Juvederm), calcium hydroxylapetite (Radiesse), PMMA (Artefill) or poly-L-lactic acid (Sculptra) come in small volumes (1 cc to up to 10 ccs with Sculptra). Thus, they are not sufficient (considering the cost) to correct these areas and more suited to the face.
Fat augmentation is a wonderful procedure for large volume augmentation. Fat grafting, however, will lose approximately half of the initial volume placed in your tissues thus, often requiring a second or even third procedure to achieve a "final" result. You also get the benefit of liposuction in the surrounding areas to help give the appearance of a more dramatic result.
Finally, choosing your plastic surgeon based upon price alone will likely leave you wishing you had done more research prior to your procedure. Nearly anyone with a medical license could perform this procedure in their office - so the onus is on you to do your research.
Have several consults, look at before and after photos and choose someone who seems to understand your desires, listens and you "click" with.