$25. The solokey 2 already used a STM chip that could support at least 20X the storage (in USB mode), but didn't activate it in their initial firmware..
Additional flash that is just as secure would be expensive mostly because other Smart Card uses don't need it, but it doesn't really have to be secure because storing resident keys could be done in a similar opaque style as a server and only really brought in to the secure context when needed.
Edit- misremembered NXP->STM and added USB as difficulty getting significant flash within the NFC powered chip is an important consideration.
Additional flash that is just as secure would be expensive mostly because other Smart Card uses don't need it, but it doesn't really have to be secure because storing resident keys could be done in a similar opaque style as a server and only really brought in to the secure context when needed.
Edit- misremembered NXP->STM and added USB as difficulty getting significant flash within the NFC powered chip is an important consideration.