Black Monks :
Stadium isn't even the most expensive building. The finance building requires a fourth bank, others do as well.
Ok let me revise my advice to the original poster based on these facts. My advice was offered in good faith by the way and wasn't designed to start an argument.
In my opinion the finance building is one of those buildings that is pretty useless after a while. You're probably just discovering this now as a level 15 player (I gleaned that info from another thread you responded to). My advice is therefore tailored to you as a lvl 15 player and not meant for a lvl 50+ player. Finance is likely one of those buildings you relied on heavily in the early stages and then gave up on because it was so expensive to upgrade for such little return, right? Same here. Not interested in upgrading that now until much, much later in the game.
The buildings that matter to me, as a player that values a team that plays well on the pitch itself are: Strat Towers (unquestionably no,1 for performance), training ground, then probably hospital if you are not a member of a facebook heal kit circuit, otherwise auction house. Inbetween that you want to focus on upgrading your snack bars first (especially if you are online a lot) or if you log in more sporadically, then the goodies shops. Also your ticket gates of course. Hence the need to spend your early gold on 3 builders - seriously do that asap. You'll find the game guides you in this anyway, you'll have to upgrade your money-makers in order to be able to afford your performance-enhancers. You do need 3 builders though to cut down on the time needed for that. if you are keeping all 3 busy all the time, you are doing it right in my opinion.
As an aside, I even had an upgrade system early on of upgrading the snack bars overnight so I lost less in income as the goodies shops come into their own over 6 to 8 hour periods of absence. I would then upgrade goodies shops during the day, when I was emptying the snack bars regularly. Now I'm less fussy as the cash flow is always really good.
ALL of those important buildings can be upgraded to the max with 3 banks and 3 alone. Fact.
My advice to you is to rush to level 15 stadium, then build your second strat tower immediately. Some then push on to level 20 for the 3rd strat tower, but I paused there and upgraded some of those buildings I listed above for a while before going on a massive push to get to lvl 20 stadium and the magical 3rd strat tower. Now I am paused again and still regret the 4th bank. Utter waste of money and time (especially as I bought the plot for it too).
Don't bother with MB, it is a much more effective and cost-effective strategy to upgrade the strat towers to get your performance bumps. All MB does is give you one off performance bumps for a lot of money - bad money management for relatively little gain. Strat towers really improve your game on a permanent basis, including friendlies so are a fantastic investment. Great money management.
Good luck to you and one last tip - train your guys around the clock (literally). That, to me, is the real secret to being good at this game. Everything flows from there - especially if you are in a good guild who help you with your strats and let you know not to buy building plots with gold early on