Operators have to pay for bandwidth of data channel based on usage. Usually signaling link is typically charged at a flat rate. SMS doesn’t cost extra for operator since it is sent over the signaling channel (Remember the flat rate for Signaling link).
In SMS, messages are sent with a “store-and-forward” mechanism meaning the message is sent only when the channel is free. The messages are sent to a Short Message Service Center (SMSC), and then relayed to the intended recipient. If the messages do not reach the recipient upon the first attempt, then the SMSC will try again. It is important to understand that SMS delivery is not guaranteed. Many messages cannot be delivered, but the delivery is called “best effort.” The amount of attempts to send a text message varies with the company.
To make the SMS transmitted over signaling link, the maximum allowed payload length is 140bytes (Others will be used for header info etc). SMS uses 7 bit characters (Not 8) leaving 128 possible character values instead of normal 256. Hence a text message contains 140*8/7 =160 characters at the max. If we use Unicode character which takes 2 bytes for each character, max character in a message will be 70.
Note: The article is written in general. There are exceptions to the concept where the operator insists the message to be sent over data channel. In this case the message will charge the operator also.