I mean, you could, but is that necessary? The main purpose of the exchange rate is for authors (and readers) to have a feeling for the prices anyway, not so much economical relevance. And as the £17 to a Galleon exchange rate, for that exact reason, is set precisely in such a way that the pricing level (taking into account the different economies) mirrors the (current) Muggle one, you wouldn't need subsidies. Compare:

£170 pounds birthday money, her parents are both dentists. IMO, fair enough. No?