I was commuting (up until last month) 100 miles a day. Did 63k in three years. Depreciation is total (BMW dealer wont accept it as a part ex with over 60k on). I have been through 9 sets of tyres (at £167 a pair - not including the two fronts I have had to send back due to faults). Servicing has cost around £180 at 6k, 18k, 30k, 42k, and around £300 at 12k, 24k, 36, 48k, and over £500 at 60k.
And now I am facing bills for a clutch, final drive, possibly a gearbox rebuild....
Compared to my wifes car which would have cost less than £200 a time for the 5 services it would have needed, £400 for a set of tyres, but would have only needed the two sets. Her insurance is nearly half mine, petrol consumption slightly worse. And her car is still worth some 35-40% of what she paid for it new.
So cost wise, no comparison. I commuted by bike because it was the only way that was viable due to traffic conditions, but the hour a day saved has to be balanced against the very real risk of having to have a month (or more) of work due to injuries - I was hit three times in two years (though fortunately I managed to stay on the bike).
IMHO a GS is not an economic proposition for 15k per anum - you need to accept this, and be willing to pay the extra over a car for the convenience / fun whatever.
PS, commuting 100 miles a day (all but 6 on the M25) for two years dam near made me give up motorcycling completely (and I have had bikes as my sole form of transport for nearly 20 years).