build lower tiers if you want to stay in this game for a long time(and promote to higher leagues,of course).if you as me then your final arena should look like-15500,6000,500,50 with the ticket prices of 18,30,170,950
of course i agree with you that investment in roster is more important than having a big arena(the best example is the present situation of AC Milan) so what you should do is to improve your arena everytime you almost fill your arena out.that would give you enough cash to invest on your roster and on your arena as well.