Where to find cheap PCB-mount SMA connectors?

As for most parts, if you order more than just a few pcs, LCSC is the cheapest source of these components (however, you need to consider the rather expensive shipping costs).

Here’s some SMA connectors I’ve used:

• Straight connector: BAT WIRELESS BWSMA-KE-Z001 0.1853€ @100pcs
• Right-angle connector: BAT WIRELESS BWSMA-KWE-Z001 0.2374€ @100pcs

Note that I don’t use them in typical RF applications, so I don’t have any information on impedance matching or accuracy etc.