A bit more verbose but you have the full power of XQuery with you. XSLT however is more verbose than that like you mentioned.

    for $price in json-to-xml(unparsed-text($file))/map/map/number[@key="price"]
    return $price+2

For the following JSON document:

    {
      "fruit1": {
        "name": "apple",
        "color": "green",
        "price": 1.2
      },
      "fruit2": {
        "name": "pear",
        "color": "green",
        "price": 1.6
      }
    }

The call to json-to-xml() produces this XML document:

    <?xml version="1.0" encoding="UTF-8"?>
    <map xmlns="http://www.w3.org/2005/xpath-functions">
       <map key="fruit1">
          <string key="name">apple</string>
          <string key="color">green</string>
          <number key="price">1.2</number>
       </map>
       <map key="fruit2">
          <string key="name">pear</string>
          <string key="color">green</string>
          <number key="price">1.6</number>
       </map>
    </map>