At £4 each then I am sure you can make them cheaper, but I am not sure you will be able to make similar to Vince's blanks anytime soon, I don't want to put you off, there are plenty of blanks you can make with little effort and experiment to make superb blanks, there are quite a few people here who make blanks, have a look at what they make.
Initially it is probably better to buy resin in 1 litre and if you decide to continue then you can buy in 5 litres. I only cast resin in the summer or if it is a really warm day because the polyester resin stinks awful and if you did in in the house, you would be in big trouble with whoever else lives with you. It is so potent it affects my breathing so have to wear a gas/vapour mask.
Peter