Don't the voters subsidise everything the government does on their behalf? Sometimes I wonder where your head is!
Hypothetically speaking, if "the voters" were electing the SNP to govern the whole of the UK then your point would make sense.
As they're not, it's pretty obvious that in the case of a Lab-SNP coalition, Sturgeon (assuming that she's actually going to abide by her parties own constitution) will attempt to win concessions for the benefit of Scotland over rUK. Given that Scotland is already heavily subsidised by rUK, I don't see how that works out in our favour.
Mind you, there's always hope that Miliband might demonstrate some backbone and refuse to do business with a party who's raison d'etre is to break up the union, even if it comes at terrible cost to their own voters.