The cheapest NTAG215 here is $0.38/pc. Cheapest NTAG216 is $0.31/pc. Both in credit card size, so NTAG215 are 25% more expensive than its big bro.
**The following are non-educated guess only**
Took a brief look at the datasheet. The difference between the 2 chips are the location of 'dynamic lock bytes' and 'configuration pages' as well as the value in CC. So in order to use NTAG216 in place of NTAG215, the data for 'dynamic lock bytes' and 'configuration pages' need to be written to the correct location. CC bits could be set to FF (refer to Amiibo article on 3dbrew) so the difference in default value does not matter.
Oh, by the way, I do not have the tools or '216 tags. So please don't ask me for it. Tagmo source code is freely available for everyone to tinker with.
**The following are non-educated guess only**
Took a brief look at the datasheet. The difference between the 2 chips are the location of 'dynamic lock bytes' and 'configuration pages' as well as the value in CC. So in order to use NTAG216 in place of NTAG215, the data for 'dynamic lock bytes' and 'configuration pages' need to be written to the correct location. CC bits could be set to FF (refer to Amiibo article on 3dbrew) so the difference in default value does not matter.
Oh, by the way, I do not have the tools or '216 tags. So please don't ask me for it. Tagmo source code is freely available for everyone to tinker with.