To me, a trophy wife is a second wife to replace the original. Usually the trophy wife is much younger and more attractive and may be more successful/intelligent (man's choice) than the first wife - and would not have been attainable earlier in the man's career, prior to the accumulation of wealth/possessions/power/fame/etc. Typically, the cost of a trophy wife is 1/2 of your assets and, if you have kids, child support.
Note: calling or introducing your first wife as a "trophy wife", whether she deserves the title or not, is a little nauseating. On the other hand, it's just cynical when you do it for the second or successive wives.
I think a key part of a trophy wife is, she only brings looks to the table. The guy, of course, only brings money/power/fame to the table. If either party should lose their primary asset, I think the other should be able to dispose of them and get all marital assets. So when the trophy wife's looks start to fade, she can be put out to pasture with no harm to the man. If the man should lose his standing, the wife should get all remaining marital assets.