There were plently of taxis available at the airport when we arrived. There is a list of taxi fares on this site here
http://www.mytobago.info/taxis02.php - it mentions these were the prices in 2003 but I don't think they've gone up much since then - certainly not to Castara anyway. It's always best to check the price before getting into the taxi. Information also found on this site states that "Official taxis carry vehicle registration plates that start with the letter 'H'"
If you contacted the Arnos Vale they may arrange for a taxi to pick you up for a similar fee. At the end of a long flight it's great to be met by someone so you don't have the hassle of organising your own transport.