how can i set up a simple budget remaining app that takes into account a set budget, multiple offices, and shows the budget remaining by office?

Create a table for "Offices" and have fields in that table for "Office Name" and "Budget Amount". Create a one-to-many relationship, so that One Office can have Many "Costs". Within that new relationship, create a Summary Field that takes a "Total" of the field where you enter the Membership Cost. Create a formula-numeric field in the "Offices" table and set the formula to:
[Budget Amount] - [Total Membership Cost]
using whatever you actually call the fields.
You will need to set the "Related Office" reference field value for all existing data so that it will include them in the calculations.
Hope that helps,
Keira