The cost is closer than most people expect. When you send mail yourself, the expense includes the stamp, envelope, paper, and any printing costs for photos. By the time you add up supplies, the per-letter cost is not dramatically lower than what a service charges, and that math does not account for your time in assembling, printing, and mailing everything.
The real reason people use InmateAid when they could just as easily send mail themselves is privacy. When you send a letter through InmateAid, the return address on the envelope is InmateAid's office in South Florida, not your home. Your address does not circulate through the facility mailroom, and it does not end up on a list of contacts that other inmates or staff might see. For some people, that is a minor consideration. For others, particularly in situations involving domestic issues, restraining orders, or simply not wanting their personal information in a jail environment, it is the primary reason to use the service.
If privacy is not a concern and you are comfortable with your address on the envelope, sending mail yourself is a perfectly fine option and saves a small amount. If keeping your personal information out of the facility matters to you, the service cost is worth it.