I wouldn't worry too much about the mess fees. They only become excessive if you join a mess that holds regular functions.
No, the expensive part of being a subaltern are the bribes. You need to keep your SNCOs' palms well greased so that they arrange for you to look good. It's hierarchy, you see. Your Troop Staffy will need to provide an incentive for his Cpls to bend the Sappers to their wills. The Sappers will do anything for a couple of beers a month, but the Cpls need to be maintained more regularly and, I'm afraid, they'll need more than just a couple of beers. Your Staffy will have further outlays - MT, Stores, etc. Organising things to be in the right place at the right time doesn't come cheap, you know. There are a lot of people to be paid off in beer, biscuits - sometimes even hard cash.
It may seem expensive - Â£400 to Â£600 per month should cover everything except major exercises - but it's a small price to pay to set you on the promotion ladder. Everything will run like a well-oiled machine and it'll be you who gets the credit for it. Just stay in your office keeping abreast of developments on ARRSE and remember to answer the phone before the third ring and everything will work out right.
It's not a long-term financial commitment. In no time you'll be a Captain and have your own subalterns to take over the costs. Then you'll only have the SSM to finance - no big pyramid beneath him, you see, they're already taken care of - and you can start to amass your fortune.
On a serious note to the OP - we're not Guards or Cav in the Corps. Mess bills aren't outrageous - it depends on the mess to a great degree, but expect to pay between Â£50 and Â£150 a month, largely depending on your bar tab.
edit: and puttees is right about answering the phone. People get upset if it rings more than 3 times. I solved this by taking over a phone line that wasn't listed on the phone directory and never telling anyone my number. Meant any calls for me had to come through Staff first - which filtered out at least two thirds of them.