Gold

From the Castlevania Wiki, the Castlevania wiki

GOLD is a statistic & a form of currency in the Castlevania series.

It usually appears as either coins or Money Bags.

It can also be obtain by selling items. The Gold ring accessory can be used to increase the rate & amount of money found. In Aria of Sorrow & Dawn of Sorrow, Soma Cruz can obtain Enchant-type souls which allow him to obtain gold as he gets hit.