Unfortunately, anything besides D6 seems to be locked behind its own individual IAP. I can't even buy one because it opens on my phone for some reason, and the app isn't compatible with phones and won't let me actually complete a purchase, so I don't even know how much they're charging for each die.
That's actually a little scummy, because the Play Store doesn't even list this as having IAPs at all. Probably because it's not a phone-compatible app, so it doesn't seem to register as having IAPs on the mobile or web view of the app's page. It feels a little misleading, considering the description says:
This powerful app brings the excitement of rolling different types of dice, including the D4, D6, D8, D12, D20, and even the D100, right to your wrist.