As part of the SMPP specifications, when we receive a request that includes more than 160 characters we must split up the messages in order to send them. They will then be reassembled into a single message on most handsets.
Twilio will bill per message, not per request. For example, if you send a message with 200 characters to a US mobile number, you are charged for $0.0075*2= $0.015. Note that special header needs to be appended to handle concatenated messages, so each segment can only contain up to 153 characters.
Messages with one or more non-GSM characters have to be sent using UCS-2 encoding and can only contain 67 characters per segment for multi-part messages. Therefore, sending a message with 150 characters that includes a non-GSM character would cost you $0.0075*3= $0.0225
See our FAQ article on non-GSM for more information.