Hey Jeff,
At this time, Custom Fields sync'd to RCE can only be used as Dimensions.
When customers do not want to use the standard amount field, they can remap a different field in SFDC to the Amount field in Marketo. Thus, Marketo will base many of its metrics off this new SFDC field instead of the Amount field.
If you are interested in doing this, please open up a Support Case.
Best,
Will