Eating enough protein food is really important. Eating enough protein food without eating the fat surrounding it is even more important still.
This does make the meal more expensive, there is no question about it. Trimming the lamb cutlets oneself means throwing the fat away. Alternatively, purchasing them already trimmed means the fat was still thrown away. The product just has a higher price to reflect that.
Consider this elegant arithmetic. The cost of a generous serve of trimmed protein roughly equals the cost of some (quite some!) serves of chocolate ripes and barbecue crisps. This month I can afford the trimmed protein.