While what Manon is suggesting is a good strategy, I do think you can combine competing and expanding your arena, if you invest some time in it. If you buy 28-32 year old players at a low price, you can sell them around the same price you bought them. You need to be patient with buying these players, as you need to wait for an opportunity to buy below market value, so by the time you sell them (before they turn 33), you can actually sell them for more or the same price (since the value drops with age).
This way, you don't lose money to them (except for the salary you need to pay, but that's always the price for competing), because if you buy a player you can sell one as well, and use the money from the sales to expand your arena. I don't have much experience and I'm not competing at a very high level yet, but so far it's working for me. It could be that it's near impossible at higher levels, but you could look to give it a try.