It depends entirely on how many authentications you have in your community. Are you requiring everyone to use 2FA? Offering it to everyone? Just privileged accounts? Then, if you exceed their starter plan of 100 auths per month (or want SMS/Voice support), you pay 9 cents for each successful auth.
If 100 users log in once successfully with authy in a month, you'd pay 9 cents * 100, or $9 (+ tax?).
To get an idea for its usage, you might consider inviting a representative group of members who would be willing to test this out for you and opening up Authy to that member group only. You can then take a look at costs/number of authentications per month from your beta group (and get some feedback on user acceptance, etc.) and then do some math to estimate what it would look like if X times more members used the system.