I suggest first designate a currency after research then we can set the value from (1-100)which causes the printing of currency.

Every civ will set value and give the different value of printing value their currency.

Thus cause the following effect:

The inflation cause decrease in the value of the currency.

It caused by:

 If you have a deal and got the payment in your currency.

Foreign trade causing an influx of your currency into your economy.

If the printing of money is increased.

If you exchange foreign currency with your currency.

Deflation causes an increase in the value of the currency.

It is caused by:

If you stop or decrease the printing of currency or destroy some currency.

If your currency moves out from foreign trade.

If you exchange your currency with foreign currency.

In the deal, if you do the payment in your currency.

When the currency is established value is according to the amount of gold coin you have.

This causes a change in price in the international market.

You will have to give the currency supply to your people so there are banks.

There can be financial companies(read the financial part in this thread) you will have to give money share/supply to these companies by setting value.

These companies can also give loans to other civs. 

These companies can only be state-run means under you.

A company from other civ can also be established and can provide you with foreign currency and you can also do the same by establishing your company in other civ and can outsource your currency.

This mechanic can give high money input.