Here's an important consideration: if you don't MAKE SURE you give your friends your Google Voice number, they will in all likelihood text your normal cell phone number (20 cents right there). If you get their text, and without noticing the icon of the program you use to send your reply--Google Voice is a classical phone handset inside a circular "call-out", the default Android program is called "Messaging" and is a smiley face inside a rectangular "call-out" (THEY LOOK VERY SIMILAR)--your outbound reply will be another 20 cents. That handles threads that begin as an inbound text to your normal cell phone number.
What about if you send out a text from Google Voice? Two things can still go wrong. Either your text pal may send a response to your cell phone (which is pretty unlikely) or they will hit reply, which means it goes to to your Google Voice number. As several here have noted, it's probably the case that you have texts set up so you get a copy to your cell phone number (another 20 cents).
Turning off this copy is not so easy. The only way I've been able to do it is with the following:
(1) Log in to Google Voice on your computer.
(2) Go to "Settings" (upper right corner), select "Voice Settings"
(3) Under the "Phones" tab, find your cell phone
(4) Select the "Edit" button
(5) There is an option called "Text Settings"
(6) Is "Receive text messages on this phone (mobile phones only)" selected? If so, turn it off! That is probably the problem.
This should solve the problem for texts that are correctly routed to your GV number, BUT REMEMBER: you have to make sure everyone uses your GV number to send you texts (or have Verizon turn off texting to your cell phone number) or you'll probably continue to pay for texts when you shouldn't!
-Matt