You could do +1 AC for 1000gp across the board, but that's a slippery slope toward complexity. That alone would double the amount of armors available.
It was just a point of interest, I guess, that fighters traditionally looked forward to one day affording plate. The most consistent solution would probably be to go the 4E route for this, and just say that plate armor is a well-established technology that doesn't cost much more than other forms of heavy armor. I think 4E had plate armor at 50gp, but that was also the edition where you could re-skin anything into anything else.
The reason to wear plate instead of (heavy) chain would be a personal preference, because you wouldn't have to move around as much in order to get the same protection out of it, or possibly out of tradition to a time when only nobles could afford plate (if you want that to be a part of your setting history). Or if you did want it as a status symbol, you could really go all-out on little artistic details in the plate, in ways which wouldn't be possible with chain (e.g. you find a suit of ornamental plate armor, which protects as well as any other heavy armor, but is also worth 5000gp as an art piece).