these are quite expensive. There are some pokerchip-looking strap locks from other brands that are very cheap.
However, if you're financially irresponsible enough to put these on all your guitars and straps like I am, you'll be able to switch around your straps super quickly without losing the benefit of a straplock.
putting them on is a bit of a hassle when you do it for the first time, but my suggestion is to use some small pliers to put on the c-clamp. otherwhise it's just a matter of pressing down on the stacked parts to create the clearance.