Yeah I'll second a GSM module for very infrequent transmits - get it to send you a text. A prepay sim card will last a long time, and as the transmitter has plenty of power available it'll be fine.
The toruble with say, a 315/915MHz system (in the US?) is, you may get the range out of it, but the reciever will have to be constantly listening, which uses up power, which means regularly recharging the portable monitor, where-as every has a phone thats always listening.