This is true on a less basic economic level than the previous poster was using but the fact remains.
pretty girl > ugly girl.
The subject is a specific girl with a particular set of characteristics to be improved with plastic surgery.
The plastic surgery makes the ugly girl prettier, thus improving her net worth if we assume pretty > ugly.
Yes she will age, but she will age with better looks than she previously would have, and will age slower assuming further surgeries, face peels, personal trainers etc.
So, in the example of this specific girl, pretty > ugly and surgery bringing her from ugly to pretty does increase her net worth.
If you drop a new engine in an old car, you increase it's net worth though you spent money on the engine.
If you paint an old house you increase it's net worth though you spent money on the paint and labor.
Money spent is irregardless to the end result of the single subject of increased net worth which is being discussed here.