If you look at the ingredients on almost any batter mix pack, you'll find it's nothing more than flour, milk powder, and salt.
For a given quantity, this could well cost you over five times more than doing it yourself.
Much better to buy a batter mix - they only cost about 20p a bag and saves you having to buy all the ingredients (especially useful if you're unlikely to use a whole bag of flour) There's also no taste difference whatsoever - supermarkets own make is as good as any!
It's actually cheaper to buy a batter mix than to buy individual ingredients, thanks philandlil for your relevant answer, I got the tesco brand in the end!