Check if a object is defined, best practice.

if(x && typeof x.y != 'undefined') {
    ...
}

// or better
function isDefined(x) {
    var undefined;
    return x !== undefined;
}

if(x && isDefined(x.y)) {
    ...
}

This will work for any data type in JavaScript, even a number that is zero. If you are checking for an object or string, just use x && x.y within the if statement, or if you already know that x is an object, if(x.y) ...

Use the guard pattern:

if (json.response && json.response.freeOfChargeProduct && json.response.freeOfChargeProduct.details) {
    // you can safely access the price
}  

This is how the guard pattern works.

if (a && a.b && a.b.c) { ... } else { ... }

The first check is "Does the property a exist?". If not, the else-branch gets executed. If yes, then the next check occurs, which is "Does object a contain the property b?". If no, the else-branch executes. If yes, the final check occurs: "Does the object a.b contain the property c?". If no, the else-branch executes. If yes (and only then), the if-branch executes.

Update: Why is it called "guard pattern"?

var value = a && b;  

In this example, the member b (the right operand) is guarded by the && operator. Only if the member a (the left operand) is truthy ("worthy"), only then the member b is returned. If, however, the member a is falsy ("not worthy"), then it itself is returned.

BTW, members are falsy if they return these values: null, undefined, 0, "", false, NaN. Members are truthy in all other cases.