Using a user input value in a simple math equation

Hi everyone,

I'm trying to add some functionality into an iPad app that will use a user input value in a simple math equation. Currently, in a View, I have a product image, description and price. The code looks like this (The last section, var text, is probably the most important section if you want to skip down)

var product1 = Titanium.UI.createView({
    backgroundColor:'#b7b7b7',
    borderRadius:3,
    width:748,
    height:183,
    top:10,
    left:10
});
 
productScrollView.add(product1);
 
var productImageView1 = Titanium.UI.createImageView({
    image:'products/product1.jpg',
    borderRadius:3,
    width:370,
    height:163,
    top:10,
    left:10
});
 
product1.add(productImageView1);
 
var product1TextBox = Titanium.UI.createScrollView({
    backgroundColor:'#ffffff',
    contentWidth:290,
    contentHeight:'auto',
    borderRadius:3,
    width:300,
    height:163,
    top:10,
    right:58
});
 
product1.add(product1TextBox);
 
 
var text = Ti.UI.createLabel({
    text:'PRODUCT TITLE\n\n$99\n\nProduct description.\n\nInstallation Charges Extra',
    color:'#000000',
    width:'auto',
    height:'auto',
    top:5,
    left:5,
    font:{fontSize:14,fontFamily:'Helvetica Neue'}
});
 
product1TextBox.add(text);

I want individual franchises to be able to input their shop rate, then make an equation that would be:

Shop Rate x Installation Time + Product Price (in this case $99)

I have an area where the user inputs their shop rate using this code

var shopRateTextField = Titanium.UI.createTextField({
    //hintText:'User name here...',
    height:'auto',
    width:'30%',
    top:60,
    textAlign:'left',
    borderStyle:Titanium.UI.INPUT_BORDERSTYLE_ROUNDED,
    keyboardType:Titanium.UI.KEYBOARD_NUMBER_PAD,
    returnKeyType:Titanium.UI.RETURNKEY_NEXT
 
});
 
var calculatePrices = Titanium.UI.createButton({
    title:'Calculate',
    width:'30%',
    height:'auto',
    top:90
 
});
 
calculatePrices.addEventListener("click",function(eventObject){
    alert(shopRateTextField.value); 
});

Does anyone know how I can make the equation above and have the result show up in a label? Specifically in var text where the $99 currently is.

I've been going at this for a week and a half and don't have much to show for it. Any help would be greatly appreciated.