While the cost per SMS does go down as a mobile carrier expands their network, it uses a separate channel from the voice and data, but the same channel your phone uses to negotiate a connection with the tower and to migrate between towers. The premium paid for SMS messages on your phone bill is due to this.
I'm not saying that the cost charged for SMS messages reflects their actual delivery costs to the provider, just that there is a real, measurable cost associated with each SMS message. Essentially, the question you're asking is "What costs more, a gallon of milk or driving to work?"
I know, my question is very much apples vs oranges. My question is something like: if I could use email as a perfect substitute for SMS messages, would I (a web startup) be able to compete in a downward spiral on price against operators?
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.
Your question isn't very answerable.
Ask a more specific or meaningful question.