jQuery .validate() plugin, additional method to validate numbers
jQuery.validator.format()
jQuery.validator has a format method
This allows you to replace the String  you created entirely.
However, this would require you to replace your 'o' parameter object with an array ( [min,max] ).
$.validator.addMethod('c_number_between',
    function(d,i,o){
        var num=d.replace(/\./g,'').replace(',','.')/1,
            // min/max assigns for context readability (optional)
            min = o[0],
            max = o[1];
        return max&gt;=num&amp;&amp;num&gt;=min;
    },$.validator.format('The value must be between {0} and {1}'));

Update:
While the docs say:

The default message to display for this method. Can be a function created by ''jQuery.validator.format(value)''. When undefined, an existing message is used (handy for localization), otherwise the field-specific messages have to be defined.

Any function that returns a string can be used so boom:
$.validator.addMethod('c_number_between',
    function(d,i,o){
        var num=d.replace(/\./g,'').replace(',','.')/1,
        return 0.max&gt;=num&amp;&amp;num&gt;=0.min;
    },function(params){return 'The value must be between {0} and {1}'.replace('{0}',params.min).replace('{1}',params.max);});