Looking at fare differences, 'Fare 1' of a return on the same route with a similar bottom line is $439.00 in CW and $14.50 in WT - so logically the difference should be $439.00-14.50 - although I guess the 75% overrides that. Then there is the issue of the dreaded 'carrier imposed charges' - which presumably would be refunded pro rata (i.e. $2000 vs $350 return, so a refund of these of $2000/2 - $350/2 ???). Taxes are the same since (I presume) these only affect the UK departure, which is the same.