In the UK, the maximum reduction in the minimum hours required for a PPL(H) licence issue is 6 hours if a PPL(A) is already held.
So, at £250 per hour for a helicopter that would be £1,500. However, a fixed wing licence first would cost, say, £5,000. For £5,000, one could have 20 hours helicopter. So, if you passed your PPL(H) in 65 hours, you would break even. As I said, I did mine in 67 hours so if I had done a fixed wing licence first, I could have saved £500.
Of course it's hard to quantify how many hours it would have taken me had I passed PPL(A) first but I really don't think it makes that much difference. Airmanship has to be learned anyway. As does radio. If someone doesn't actually want to fly fixed wing, what is the point of doing it and spending the extra money for a licence that may well lapse because the motivation isn't there to maintain it and keep up the hours?
Cheers
Whirls