Yes, there is a cost in both directions, and it is worth understanding what you are actually paying for before deciding whether it makes sense for you.
First, these are not emails. InmateAid uses the US Postal Service. When you send a letter, it is physically printed and mailed to the facility. When your inmate writes back, they address the reply to InmateAid's office in South Florida, where it is received, scanned, and posted to your account dashboard. You pay a small fee to access that scanned letter.
What that cost buys you is convenience, reliability, and privacy. Your home address never appears on anything going into or out of the facility. You send from your phone or computer without printing or stamps. The return letter comes to a secure account rather than your mailbox. For people who move frequently, want to keep their address off the inmate's paperwork, or simply want a centralized record of correspondence, those things have real value.
If you are comfortable using your home address and standard postage, you can always write and receive letters the old-fashioned way at no cost beyond a stamp. InmateAid is an alternative for those who want the added layer of convenience and privacy, not a requirement.