Just got a reply from Vodafone NZ about the cost of receiving SMS reminder from Google Calendar. I originally thought it might cost me 20cents for every TXT I received...but it turns out that its all FREE!

Dear David
Thank you for your email.

I can confirm that the Vodafone Network does not have an incoming SMS charge. As this is not a Vodafone service, we are not best placed to advise you of Google charges. I have however looked at the Google Calendar site and can see that they do not charge for message notification.

Google Help Centre: Is receiving reminders by SMS free?

http://www.google.com/support/calendar/bin/answer.py?answer=37173&useful=1&show_useful=1

Q. Is receiving reminders by SMS free?

A. Google Calendar is a free service and Google doesn't charge you to receive notifications. However your mobile service provider's standard rate for receiving text messages may apply. If you have any questions about the rate, please contact your operator for more information.

Great news! Now I can set lots of SMS notifications on my calendar :)