My plan is to max out my arena (10000/5000/500/50) - which i think I can do in about 2 seasons. The arena, unlike a player or a staff member or w/e, is not a depreciating asset.
Put another way if I buy a good player, 5 seasons later, he is worth less. The arena is not. Plus, it doesnt cost anything to maintian once i max it out.