I have done this done before for university project. I have your project ready. Seriously speaking, you can send 6 sms per second using solution i modified for you.
And It will use your own cell phone to send sms. So it will cost as low as your GSM Subscriber allows you. Normally we have many sms packages where you can buy a best for you. You might need a Nokia cell phone for this purpose, because I have tested it on nokia phones its working perfectly. Window XP,7,8,8.1 is required.