I've been looking for a cheap texting solution, out of curiosity. Tried a few applications that claimed they were free but they didn't work. Found FishText, allows the user to send text messages internationally for just over 1p and to the UK for 2p. Your network provider will charge for GRPS access, but this should be around 0.002p per message sent (based on £2/MB which most networks charge). If you have a data plan you could save a fortune.
You also get 20p free credit when you sign up (=20 free UK messages)
If anyone knows a cheaper/free service please add to the thread, I've tried about 10 over the past week and couldn't find a cheaper or reliable one.
Check your tariffs as every tariff will have a different GPRS proposition & overage rate. This could end up costing more than your standard SMS overage.