Reading JSON in Google Sheets with Google Script

I am currently pulling data from an external API to Google Sheets and wanted to get a certain part of the data to show from the JSON. The only part that should be added to the cell is $440.00 but currently it shows {"data":[{"cost":"$440.00"}],"success":true,"totalNumRows":1}. Is there a way to just pull that?

// Add menu to run program
function onOpen() {
var ui = SpreadsheetApp.getUi();
ui.createMenu('Get Crobo Cost')
    .addItem('Display daily cost', 'callCrobo')

// Function to call Crobo
function callCrobo() {
// Call Crobo daily spend
var apiKey = '****omitted****'

var response = UrlFetchApp.fetch("" + apiKey + "&start_date=2018-03-12&end_date=2018-03-12&field[]=Stat.revenue")

// Parse to JSON reply
var json = response.getContentText();
var obj = JSON.parse(json);


//Gets hold of ActiveSheet in current workbook
var sheet = SpreadsheetApp.getActiveSheet();

// Set value in cell A1
sheet.getRange(sheet.getLastRow() + 1,1).setValue([response]);

The logger's current output is:

[18-03-13 17:22:02:377 EDT] {"data":
[18-03-13 17:22:02:378 EDT] undefined

Thank you