The question here is about the amount of money 'you pays'. An ATR 72-600 could take 55ish pax on a restricted load basis from an extended runway, with projected figures of 3% less fuel burn compared to the existing Dornier operation, with a reduction in subsidy from £2m to £1.3m per year at current figures. The runway and terminal, which were built in 1967, need rebuilding anyway. The runway was resurfaced in the late 1980s and in 2006, and is now crumbling to an extent that it requires regular and expensive repairs. The fire station has asbestos in its roof. It may be that it would 'only' cost an extra £8-15m to extend the runway, build a slightly bigger terminal, etc, vs to rebuild to the same length.
The other side of the coin are the political considerations. Given the size of its structural deficit, the States of Guernsey mayn't be willing, in the future, to make this level of investment in Alderney - so it's not an issue which can be returned to simply or reliably at a later date. The political will is definitely there at the moment (if the numbers are right) - but it may evaporate as Guernsey's tax debate focuses more on spending cuts, the constituent members of P&R change, and/or Peter Ferbrache ceases to be president of P&R. Hydrogen and battery powered aircraft, even of Dornier size, are likely to require longer runways than conventionally fuelled versions for take off. So, even if the ATR seems a white elephant in a decade, there is a significant element of future-proofing in option C.
Much will rest on the business case presented to P&R at the end of the month, in terms of the opportunity cost and the pay-back period of the investment. This is a decision which will determine a lot for the future of the island, whatever decision is made, and it will be very interesting to see the figures.