Best practice suggests the use of controlled vocabulary for the currency attribute, such as the ISO 4217 list of currency designators.
<elementSpec ident="price" module="MEI.header">
<desc xml:lang="en">The cost of access to a bibliographic item.</desc>
<classes>
<memberOf key="att.common"/>
<memberOf key="att.bibl"/>
<memberOf key="att.lang"/>
<memberOf key="model.titlePagePart"/>
</classes>
<content>
<rng:zeroOrMore>
</rng:zeroOrMore>
</content>
<attList>
<attDef ident="amount" usage="opt">
<desc xml:lang="en">Numeric value capturing a cost. Can only be interpreted in combination with the
currency attribute.</desc>
<datatype>
<rng:data type="decimal">
<rng:param name="pattern">[0-9]+\.[0-9]{2}</rng:param>
</rng:data>
</datatype>
</attDef>
<attDef ident="currency" usage="opt">
<desc xml:lang="en">Monetary unit.</desc>
<datatype>
<rng:data type="NMTOKEN"/>
</datatype>
</attDef>
</attList>
<remarks xml:lang="en">
<p>Best practice suggests the use of controlled vocabulary for the currency attribute,
such as
the ISO 4217 list of currency designators.</p>
</remarks>
</elementSpec>