No, you will not be charged any extra just for using biteSMS. It has nothing to do with your texting plan, all it does is make sending and receiving simple and easy. If you ever to go over your texting plan and got charged, it would be because you went over your texting limit............Not because you used biteSMS. I use it every day.
If you use no more then 200 sms, then nothing about biteSMS will change that. Like I siad, It only effects the way you handle SMS/MMS............Not your actual plan.
In fact, biteSMS can reduce the number of SMS by turning on the Accents/Diacritics option, allowing you to extend the number of characters allowed in one SMS before it becomes 2 messages when using special characters. biteSMS is the "best" messages tweak in Cydia IMO.