The cost is all about scale. an email is going to cost whatever your metered rate is for the traffic (well, technically 2x the metered rate, once for incoming, once for outgoing traffic). For Google's infrastructure and typical bandwidth usage, the cost trends to zero. An SMS message is sent as a packet of data in the control channel to your phone. The cost of an individual SMS message is not as easy to determine as the cost of an email. Your phone exchanges packets of data on the control channel whether you're receiving (or sending) SMS message or not. The SMS message is an additional piece of data that is sent over a constrained channel. In most areas, the towers will be able to easily handle the peak usage of the area without any degradation of service, meaning the towers can easily handle the traffic. However, unlike email, the more SMS messages are sent, the more expensive each individual one is since the tower has to be able to handle that number of simultaneous control channels.

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?"