This file is used by GEOSCAPE.EXE and it's structure is very simple. Every record is a 4 byte signed long integer. Probably the most useful offset is the first 4 bytes where your current money is stored. The rest of the bytes are used for the Finance graphs: Expenditure, Maintenance, and Balance (the others are stored elsewhere).
Note: This file is only updated once a month, except for the first four bytes.
Note about signed long integers: Keep in mind the game stores the values in little-endian format, so when viewing in a hex editor they will look backwards. The highest bit, (being the first bit in the last of the 4 bytes) determines if the number is negative. Thus FF FF FF 7F is the highest positive value (in the neighborhood of 2.1 billion) and 00 00 00 80 is the 'lowest' possible negative value.
(Offsets in decimal)
00: The most useful and probably the most hacked, these 4 bytes hold your current funds.
04-48: This is the expenditure for the months (exactly 12 of these 4 byte values) going in order January to December. The graph is adjusted to the current date so these essentially contain a years worth of data. 52-96: Same as above only for Maintenance 100-144: Same as above only for Balance (hence why you will sometimes see similar numbers here to your funds)