A money field supports/depends upon a currency and that introduces the question of what exchange_rate to use!
The simple answer is 1.00000 meaning that the currency gets 'traded' on par - which will satisfy the 'one currency' use case (commonly exchange_rates are listed with 5 decimals - but some currencies requires 7 or more decimals to allowing an accurate calculation, like the Afghani, currently traded against the USD at 0.0206954). One currency use cases could be set using a simple configuration option - like config.money_rails.one_currency_only = true
A somewhat more complex use case is the 'two currencies' application where all money fields are listed (and calculated) in a 'preferred' currency but has to be presented in a secondary currency as well. Some EU-countries have not implemented the Euro as their national currency, like Poland. Their national currency has an almost fixed exchange_rate towards the Euro - it sees fluctuations but only marginally. Money fields are rarely converted but even thoug