The $amount field in our $transaction and $create_order events should be sent in micros. This allows us the granularity necessary to compare different currencies.
In general, the largest unit of currency (Dollars, GBP, Euros) should be 1 million micros. Then the smaller units of currencies should be sent over as the appropriate amount (1 Cent = 1/100th of a Dollar so it is 10k micros). For example, 1 cent = 10,000 micros. $1.23 USD = 123 cents = 1,230,000 micros. So, "$amount" : 1230000.
When working with currencies without fractional denomination, continue to use micros for the $amount field. Multiply the smallest unit of the currency by 1,000,000. For example, though the Japanese Yen does not have fractional denomination, use 1 JPY = 1,000,000 micros. So, "$amount" : 1000000.
For Bitcoin, send in units of microBitcoin, where 100 satoshi = 1 microBitcoin = 1 / 1,000,000 of a Bitcoin. So, if a user used 1/2 of a Bitcoin, you would send 500,000 in the $amount field.