So broadly speaking (and I quite understand that you probably don't want to put the specific algorithm out there!) if the game balance for a coin goes down, will the corresponding AM value against that coin go up or down?
It's a tad complicated, essentially the game will go through and add up the monetary value of all game balances and add them together. This total is then divided by 1,000,000 for the total amount of AntiM to get the value for each AntiM.
If just one of the coins offered in game increases or decreases then it will affect the overall worth of AntiM.
For instance, the total worth of all game balances provided by me right now is $1,159.99 so you would take that and divide it by 1 million to get the value of each AntiM which would be $0.00115999.
If you have 100 AntiM you could trade it in for $0.115999 worth of any currency or split between multiple currencies for that amount if that is what you want.
Long story short, AntiM values are tied directly into overall value of the coins available for exchange. If the game balances values go down then the value of AntiM also goes down, if it goes up then AntiM value also goes up.