Preliminary Analysis
~~~~~~~~~~~~~~~~~~~~

Bytes 52-54 seem to be the location where the credit is stored.

Experimental data yields this:

Both $4.00 cards:
00 3E 80
003E80 in decimal is 16000 in decimal.
16000 / 4000 = 4.00

A $5.00 card:
00 4E 20
004E20 in decimal is 20000 in decimal.
20000 / 4000 = 5.00

A $6.00 card:
00 5D C0
005DC0 in decimal is 24000 in decimal.
24000 / 4000 = 6.00

A $0.00 card:
00 00 00
00000000 in decimal is 0 in decimal.
0 / 4000 = 0.00

A $0.10 card:
00 01 90
00000190 in decimal is 400 in decimal.
400 / 4000 = 0.10


Since the cards use 3 bytes to store the credit and 4000 seems to be
equivalent to $1, we can perform this calculation to find out the
maxiumum amount of money a card can hold:

FFFFFF in decimal is 16777215.
16777215/4000 = 4194.30375.

Therefore, the largest amount possible would be $4194.30, which would
be represented on the card by FF FF F0.