In LabsMobile only sent messages are charged. There is no other cost of high maintenance, support, etc.
It is necessary to make purchases or top-ups of credits that will be added to the balance of the account. As the sendings are made, the credits corresponding to the cost or unit price of each message sent will be deducted.
The unit price of each SMS message is calculated based on the destination country, length and type of message as explained in What is the price of an SMS message?.