It is hard to answer the question since you want to stick to a budget on security. Many people recommend at least 10 percent of the total price of the bike on locks and security. Cable locks can be very easily cut, though u-locks can be a bit harder. We need to know where/what general area the bike will be to gauge the type of security you need. If you live in a big city, you would probably be best served by getting a beefy chain/lock combination. If you live in a more rural setting where thief is not a large concern, you can get away with a cable lock so people simply don't walk off with the bike.

With that said, I don't think I've seen any cable lock that has lasted more than 10 seconds to a cutting device, be it moderate to small bolt cutters, to simply wire clippers.

I live in a big city so that means I need some pretty good locks. I have from time to time locked my bike up with locks more expensive than the bike itself since I wanted the bike to be there when I got back to it.