Personally I use the simple pricing scheme and looking at billing page, I pay around ~$0.35-0.4 monthly for 5 domains, with 4 explicitly set email addresses and catch-all for all domains to a common mailbox. Also I must state again, there is quite little traffic on all.
Usernames on shared domains:
1 to 6 letters: $0.20 per user per year 7 to 12 letters: $0.05 per user per year 13+ letters: $0.02 per user per year
WTF?