That depends on the material and the type of printing. The lowest cost would probably be FDM PLA. MFJ nylon or SLA resin will result in a more usable surface. A resin cast version seems to be the best quality at the DIY level.
For the lowest cost alternative what is a reasonable estimate? Are we talking about USD 1 per key or maybe USD 5 or more? I guess I could find out if I tried to place an order at one of the mentioned services but it wasn't obvious to me how to do that without first signing up, actually customizing a set of keycaps and starting the order process.
Resin is in the order of magnitude of $30 per kg, so I'd assume the materials cost would be < $10 if you already have a printer.