Simming doesn't make a difference at all no
The amount you get depends on their positions, Forwards and Wingers are the two most expensive positions in the game.
Defenders are normally relatively cheap in comparison, and Goalkeepers can be around £30m maxed out I think.
When players hit over 30, the only panels that go down are keeping, movement and strength (or physical can't remember the name) panels.
Their actual skills like shooting, passing, crossing, defending and such don't drop.
Keepers can normally stay a few extra years because they don't need as much updated to keep their skills high. His value will probably peak at around 30 years old, the same thing will happen with Howard like it did with Cahill, their values and stats drop a heck of a lot after 30 years old and if you keep them for too long you can sell for thousands instead of millions :-/
The valuation system is a bit borked to be fair, players in the high 70's are still only a few thousand pounds when as soon as you get to about 82 or 84 overall they're worth millions

.com