This was a commonJS module we played around with for node.

mustacheMathAddOn.js

////
// Math Support
//
// This is an an awesome addition to mustache.js to 
// add support for basic math operations. No evals!
//
// Examples:
//
// 2 == {{#math}} {{four}} / {{two}}  {{/math}}
// 20 == {{#math}} 100  / 5  {{/math}}
// 200 == {{#math}} 100 * {{two}}  {{/math}}
// 603 == {{#math}} 3 + 100 * {{two}} * 3 {{/math}}
// 406 == {{#math}} {{two}} + {{two}} * {{two}} + {{two}} * 100 * {{two}} {{/math}}
////

var mathRegex = /\s*(\d+)\s*(\*|\/|\-|\+)\s*(\d+)\s*/;
var mulDivRegex = /\s*(\d+)\s*(\*|\/)\s*(\d+)\s*/;
var addSubRegex = /\s*(\d+)\s*(\-|\+)\s*(\d+)\s*/;

var applyMath = (function() {
	
	var operands = {
		"+": function(a,b){ return a+b; },
		"-": function(a,b){ return a-b; },
		"*": function(a,b){ return a*b; },
		"/": function(a,b){ return a/b; }
	}
	
	return function(str, digit1, operand, digit2, offset, s) {
		return operands[operand](parseInt(digit1, 10), parseInt(digit2, 10));
	}
	
})();

exports.math = function math() {
	return function(text, render) {
		var eq = render(text);
		while (eq.match(mulDivRegex) !== null) {
			eq = eq.replace(mulDivRegex, applyMath);
		}
		while (eq.match(addSubRegex) !== null) {
			eq = eq.replace(addSubRegex, applyMath);
		}
		return eq;
  	}
}