Economy class discounted return fares vary, typically from around £500-£550 in low season to over £800 in high season.
British Aiways fly from Gatwick to Port of Spain, Trinidad, daily except on Saturdays and Wednesdays and from Gatwick to Tobago on Mondays and Fridays.
Virgin fly from Gatwick to Tobago on Saturdays and Tuesdays.
US airlines offer daily services from Heathrow via their US hubs, for which you must register on the ESTA wesbite https://esta.cbp.dhs.gov/esta/ at a current cost of $14 (£10pp).
Flights to T&T get very busy at Carnival. Flights between Trinidad & Tobago can cost from as little as £60 return.