If you have an older door that doesn't need the security, or if you can solder, you can get a Zen17 for $30, and a ZSE43 for $25 both on sale currently at Zooz - The Smartest House
They have been priced slightly lower at various times in the past, but the sale price now is still pretty good. Not quite the $40 that Rick quoted above, but still a decent price.
If you need the security dongle (which you don't always) that would add another $25.
You could always just go with a Zigbee Fingerbot for about $25 or even less on Aliexpress. The link below show some examples of this, and other alternatives.
