Formula and cows' milk have about the same calories volume for volume. However, formula is fortified with extra vitamins and minerals. It also contains less protein than cows' milk making it easier for smaller babies to digest. I'd be inclined to do as your doing and continue with the formula until the twins are aged one corrected, but I have no scientific/basis for saying so! Could your HV advise you?
I have no idea why Farleys is so much cheaper. I use C&G Premium which is just under £7 a tin now. I haven't heard that one needs to supplement if one uses Farleys- again, I'd be inclined to ask the HV. There are laws regarding the composition of infanmt formula and Farleys must meet these, although I know that C&G/Aptamil etc make a big play of some of their additional ingredients (e.g. prebiotics) so perhaps that is why they market them as a more expensive product, Whether there's any real scientific basis for their claims, I wouldn't liek to say!