What a travesty . I make good cupcakes so invariably bring a batch for work events, but I always fork out for butter for the buttercream swirls on top. I buy a reasonably quantity whenever a supermarket offer works out a £1/block then freeze it. Having said that, I baked lemon cupcakes for work's MacMillan coffee morning so just topped them with stabilized whipped cream which was even cheaper as I found a pot in the quick-sale-fresh-section of the mini supermarket near work
Aldo and Lidl have perfectly good butter for £1 ish per pack all the time.
I just paid 98p a block for unsalted butter today.
I use stork sb tub in my cakes and buttercream for the icing, some people have absolutely no idea when it comes to cakes. At last years Macmillan do another mum complemented me on my cake saying it smelled of eggs. Another looked puzzled and said 'how do you get it to do that?' Doh, 3 very fresh eggs in it!!!
I made a batch of fairy buns (I refuse to call them cupcakes) recently. I made half with butter and half with Stork tub margarine. Everyone preferred the Stork ones. The butter ones were a little browner, but tastewise I couldn't tell the difference.
Mary Berry recommends Stork tub as well.
I use salted butter for buttercream as I find unsalted makes the buttercream too sickly, but it really is a case of personal preference. I just use whichever butter is cheapest.