The way they calculate it is a little misleading.
From AWS[1]: Provisioned IOPS is charged by the amount you provision in IOPS (input/output operations per second) X the amount of days you provision for the month.
If I understand the above correctly, the estimate of $10/month would actually be:
$0.10 * 30 days * 100 IOPS = $300/month (In USEAST)