In terms of paying for it as a user

SMS is more expensive to send. It normally costs around £0.03 per thousand to send emails when it costs around that to send that for 1 sms.

In terms as a provider

For GMail they tend to store an email at least 6x to make sure you never lose something. This means the cost of an email is actually 6kb plus all the cost to store, replicate and the electricity that goes with it. While with the SMS the provider is really cheap as it doesnt use that much bandwidth to send and receive it.