Having a second car can be a great thing, as wear I now live near Moss Side in Manchester (And not having off street parking) I would not dream of leaving the ST anyway near here. (Not the Ideal place to live, but have to do it for work!!)
I'v got my self a 1.2 Corsa for running around, and leave the ST in my parents garage for the weekends when I go back.
You appreciate a car soo much more when you havent driven it all week.
As for the costs the corsa gets 60mpg on the motorway when my focus would only get 26-28mpg, so the saving are massive if you do the millage I do! (Plus I put regular fuel in the Corsa which saves a few bob too.) Plus with the rate I get per mile at 23p, I can pay for the fuel and insurance on the company as its prety low (Insurance that is). Plus the money I'v saved has gone on the mods i'm having fitted at Profile on the 14th (Anyone going around then??)
Additionally I put 12.5K on the ST in 9months, which has now slowed to around 500-600 per month. I'm not bothered about putting miles on my car, just at the rate I was going I would have had 40k + at around 3 yrs, which judging by most of the cars sold on this forum is very high.
You just need to do the maths as people have said and see if It makes financial sense to run a second car, everyone's situation is different and you are the only one that can make the call.
It does sound like you have made your mind up anyway.