Some use their nephew who just started art school. Changing copy or repositioning art at this stage is a matter of computer time and labor. Unless you are asking for a major overhaul or new execution, it is reasonably easy to make changes at this time. Proofreading cannot be overemphasized. More often than not, the person who wrote the copy is not the right one to proofread it.
Some companies reduce their photography or illustration expenses by reusing shots again and again.
If you do this, be sure to make arrangements to buy full rights to the art from the stock photo house, photographer or illustrator to avoid disputes in the future. Arrangements made with models and their agents can vary widely, as well. Prepress costs will vary based on color or black and white, size, intricacy of package design, use of art work and even the format on which it is received by the people doing the work. It can be an adjustment of the overall color balance, the need to lighten an area that is too dark, or the necessity to wipe out some of the background. It may also be cropping of a photo to adjust to the space.