I'm trying to think about how you could offer more reassurance. Off the top of my head,

- Offer a calculator so that people can estimate their own cost. A small improvement, probably not significant impact.

- Offer a customer to enter their current IMAP creds, give an accurate estimate based on past usage. A big improvement, but which would require significant trust from the prospective client. Hard sell.

- Offer a guarantee of some kind. "If you don't come off cheaper with Purely, we'll make up for the difference". Reassuring, but increases your risk.

- Up the price of storage, and eliminate the price of traffic. Storage is probably an imperfect but adequate proxy for total usage price, given that the pattern nowadays is to archive rather than delete (non-spam) email.