Of course everything will vary. You might try working things like upgrading computers, software, and office equipment into a monthly budget by putting aside a little each month for those things. This is very rounded off, but it's a start:
Office rental/ monthly fee: $1,000/mth
Internet and internet marketing: $100/mth
REALTOR dues: $100/ mth
E&O Insurance: $50/mth
Marketing: $200-$2,000 per month
Postage: $100 per month
Gas: $300/mth
Cell Phone: $100/mth
Software/ Technology: $50/mth
Office equipment: $50/mth
Client lunches: $200/mth
You might also want to factor in your insurance costs: health, life, disability... Of course you will also have expenses directly related to each listing (flyers, lock boxes, additional marketing, etc.) I'm sure I left out other things, but hopefully everyone else will help fill in the holes.