Variable Load
I don't disagree with your figures up to the 87.5 mln but that only leaves 12.5 mln .
You divide that figure by 5 thats 2.5 mln a year and divide it it by 4 bases thats 625,000 a year per base for overheads, insurance, mobilisation, base costs, etc.
I think you'd be lucky to get the insurance for 2.5mln a year for 7 aircraft operating SAR!