I can taste the difference between butter and marg if it's not 'mixed in' but having read Delia's book on the different methods of making cakes, she recommends marg as it makes the cake lighter than butter, so I do as I'm told!
I can't necessarily taste a difference between vanilla essence and the real thing, but its a long time since i used extract so I may be able to these days - however, I like to be able to see the little black dots so I use vanilla paste, which is easier than faffing about with pods, but gives you the black dots.
I use free range eggs from an animal welfare POV rather than for taste. And have never had a problem using supermarket basic flour etc.
I use Belgian chocolate buttons, just because its cheaper to buy a bulk bag of them from Amazon, than lots of smaller bars from my local CoOp, which you then have to spend time breaking up.
I'm generally pretty cheap about shopping, including food, and I always get loads of compliments on my baking. It's more about the techniques and using good flavours together for me.
I recently made some of these to sell, using very basic ingredients - water, butter, eggs and bread flour for the choux pastry; milk, eggs, cornflour, butter and vanilla for the fillings and then sugar/glucose syrup or chocolate for the toppings. The whole thing costs less than £10 to make (although many hours of work!!) but it tasted amaaaaaaazing!