Thieves aren't stupid, they know how much common bikes cost new and they know what the going rate is for stolen ones sold for cash up the top of the Lane. The attractiveness of the bike causes the theft risk, not the fact that the lock looks expensive.
If you were to lock up your £500 hybrid with a £100 lock next to one of my skip rescue bikes locked with the same type of £100 lock, I'd bet a month's wages on it that any thief would try to nick your bike in preference to taking mine. If you want to minimise risk, then don't use a bike of any value greater than the price of a round of drinks. if you are leaving it on-street. If your bike doesn't offer a thief any profit for their efforts, they won't bother trying to steal it or the parts off it.