One ways back to Paris (Air France) were always the best I could find, via a French Island after a quick trip on LIAT. You may get a good last minute flight on a charter if they have space but you wont be able to pre book as they keep the seats for the packages.
I think it depends a lot on the date of the return. Quite a few folk stay there for Christmas and the return flight becomes a bit more expensive. It may be cheaper to leave a week or so before Christmas since the flights are likely to be busy inbound, but less so outbound. I think we paid about £400 each for the single back on 27/12. Can be cheaper to go via Castries and on a hopper to say Antigua or Barbados and catch a flight back from there (as opposed to Vieux Fort). The WCC have a few deals which they announce in Spring(ish) but you need to be quick to catch them as the allocation runs out fast. We couldn't book the dates we wanted via them. Travelocity messed up (i.e. cancelled) our tickets due to some 'computer error'. They sorted out new ones but just 2 hours before departure, flight costs then across airlines were the same price as we had paid, having booked 6 months before! And there were quite a few seats on both Virgin and Carib-star/BA.
I think the cheapest are from Martinique to Paris about 2 a day - French Charters costing around a couple of hundred and that includes the 'local' flight from where ever to Martinique. The big problem is the Paris end - Train is v expensive.
There were excellent cheapies from Tobago - about £250 on the charters - the travel agent at CruiseInn in Trini booked them.
As far as I can remember, I just went to the travel agent at Rodney Bay marina and booked one there. Must have been a reasonable price or I would have taken more trouble over it.
So if I waited to book the flight once in the Caribbean, am I likely to get a good deal or to end up having to pay scheduled fare?
Don't you just hate the way airlines screw you on one-way flights. I'm glad to see that budget airlines with single ticket sales are expanding but I wonder when Easyjet will move beyond Europe.