In fact, "small price" is practically an understatement. The most basic strap locks are simple rubber rings that slip over the stock button on top of the strap, and they're so affordable that Fender is practically giving them away in their Strap Blocks 4-Pack. You could also try replacing your stock strap buttons with something like the D'Addario Planet Waves Elliptical End Pin Strap Button. Or, if you want the maximum level of protection for your instrument, there's no substitute for a mechanical locking system like the Schaller Guitar Strap Locks and Buttons (Pair) or the Warwick S-Security Strap Locks. These models lock on tight and use mechanisms designed to prevent accidental release, so your strap will always stay put.
Without some sort of strap locking, it's all too easy for a bit of slack to make your strap pop free of its buttons. But even the most basic types of strap locks will make that much less likely, and with the highest-end options, it's virtually impossible. Consider these an easy insurance policy - an accidental drop always seems unlikely to happen until it actually does, and then it's too late. Protecting yourself from that event in advance is just plain common sense.