You can easy do it with help of Conditional Styles.
The sample
The test csv datasource:
name,price,discount_price
Shirt,"100,00","90,99"
Trousers,"80,00",
"Sun glasses","199,99","120,00"
The jrxml file:
<?xml version="1.0" encoding="UTF-8"?>
<jasperReport xmlns="http://jasperreports.sourceforge.net/jasperreports" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://jasperreports.sourceforge.net/jasperreports http://jasperreports.sourceforge.net/xsd/jasperreport.xsd" name="two_textfields_one_on_other" pageWidth="595" pageHeight="842" columnWidth="555" leftMargin="20" rightMargin="20" topMargin="20" bottomMargin="20" uuid="cb5d2807-1ef9-4ffb-8df8-fcb781054bb0">
<style name="priceStyle" hAlign="Left">
<conditionalStyle>
<conditionExpression><![CDATA[$F{discount_price} == null || $F{discount_price} == 0.]]></conditionExpression>
<style forecolor="#3333FF" hAlign="Center" vAlign="Top"/>
</conditionalStyle>
</style>
<queryString>
<![CDATA[]]>
</queryString>
<field name="name" class="java.lang.String"/>
<field name="price" class="java.lang.Double"/>
<field name="discount_price" class="java.lang.Double"/>
<detail>
<band height="40" splitType="Stretch">
<textField isStretchWithOverflow="true">
<reportElement uuid="93ad241e-5257-4664-b3ab-f6448f786775" x="0" y="0" width="114" height="20"/>
<box>
<topPen lineWidth="0.25"/>
<leftPen lineWidth="0.25"/>
<rightPen lineWidth="0.25"/>
</box>
<textElement markup="styled"/>
<textFieldExpression><![CDATA[$F{name}]]></textFieldExpression>
</textField>
<textField isStretchWithOverflow="true">
<reportElement uuid="6b53f6c6-5d48-4879-8934-d446150e1375" style="priceStyle" positionType="Float" x="0" y="20" width="114" height="20"/>
<box>
<leftPen lineWidth="0.25"/>
<bottomPen lineWidth="0.25"/>
<rightPen lineWidth="0.25"/>
</box>
<textElement markup="styled"/>
<textFieldExpression><![CDATA[($F{discount_price} != null && $F{discount_price} > 0.) ? "<style isStrikeThrough='true'>"+ $F{price} + " $</style> " + $F{discount_price} + " $" : $F{price} + " $"]]></textFieldExpression>
</textField>
</band>
</detail>
</jasperReport>
In this sample I changed the forecolor of text (make it as Blue) and the text's horizontal alignment (make it as Central, the default is Left) in case when the discount price is absent (not set or equals 0.0).
The design in iReport looks like this:

And the result will be (via preview in iReport):

Note:
For better formatting I have used the markup syntax in my sample. You can also read about styling in this post