If you want to save a little money, fortunately all you need is a repeating device that supports “beaming” in order to talk to a zwave lock. Any zwave smart plug, light switch, or relay that supports beaming should work exactly the same as the single purpose range extender in this regard. 
Way back in Z wave generation three, we needed the range extenders. But by generation four, pretty much all Z wave devices already broadcast at the maximum allowable strength, so you didn’t need the single purpose extenders anymore. Repeating capability is already built into most Mains powered Z wave devices. So it’s usually a little more cost-effective to get a dual purpose device, like a smart plug. You can still use it as a smart plug, and it will perform all the same repeating functions.
The exception is if you need a repeating device that will switch to battery power during a power outage. These won’t help in a smartthings setup unless you also have your hub on a UPS, but you do see them in security systems that have their own battery backups like Ring security. 
You make a very good point about the importance of beaming repeaters for locks. They can solve a lot of reliability problems in a zwave network.
FAQ: why would I need another beaming repeater if my zwave lock is already close to my hub?
Fortunately, that’s not an issue with a sensor, so any zwave repeating device will work. Choice is good.
BTW, if you want to confirm that any particular Z wave device supports beaming, this will be listed on its conformance statement at the official zwave alliance site:
https://products.z-wavealliance.org/
For example, here’s one for Zooz’ newest smart plug:
