You will not owe customs on the X-box, but you may need to deposit the VAT on it.
According to the Israeli Tax Authority guide for tourists, tourists are not required to declare items whose value doesn't exceed 200 USD (which your Xbox doesn't fall under).
As well there is a category for "others":
OTHER USED AND PORTABLE ITEMS WHICH MAY BE BROUGHT THROUGH THE
Typewriter, Camera, Movie camera, Video camera, Personal computer, Laptop computer, Television receiver, Tape recorder, Radio, Binoculars, Jewelry for personal use, Musical instruments, Record player, Stroller/baby carriage, Tent (only one allowed), Camping equipment, Sporting. Equipment, Bicycle (only one allowed, and non-motorized) and Similar travelers' equipment.
Since computers and televisions are on the list, I beieve that your Xbox (as long as it's not new) will also fall under this category, but it does not explicitly mention video game consoles, so there may be some room for judgement by the customs personal in your case.
In any case, however, you will only need to pay the VAT, as there is no custom on game consoles in Israel, which is 17% (68 USD). And even then, you will not pay it but only leave it in deposit, and will receive the amount back as long as you'll take the Xbox with you when you leave:
If you bring in: a new personal computer, a new VCR, a new video camera, or other similar items, you must leave a guarantee equal to the taxes due on the item. The guarantee must be [...] made in foreign currency, using cash or one of the following credit cards: Visa; EuroCard;
American Express; and Diners'Club.
B. THE GUARANTEE - RETURNING OR FORFEITING
The guarantee will be returned to you if you take out the items on time; otherwise the
guarantee will be forfeited.
So in any case, at the end of your trip, you should have no expences on the Xbox.