I am on Talk Lite and have not used Talk this month, however I was still charged $1 for usage?
The cost for the Talk Lite subscription is $0 per agent, however if you have an active phone number on the account you will be charged each month based on the number.
The price of a phone number depends upon the country from where it was purchased. For example local numbers from US and Canada will cost $1, while toll-free will cost $2. For a full list of prices by country, see Zendesk talk number availability and pricing.