Rent for this is 125 Baht per month or a deposit of 1,800 Baht. If you visit their website, there is a form (click here) where you can register for the set-top box. Unfortunately its all in Thai so you will need to find someone to help you if you cannot read it....
Save the cash for your next trip. like to have cash on arrival and not have to dart for an ATM at the airport. So in any country, with any currency, that I’m likely to visit/use again I don’t turn back any excess cash before leaving. I keep Euros, Pounds, Baht, ...