If you squeeze it, you get about 30 grams of liquid+pips, and about 25 grams of liquid, so making lemon-juice out of lemons is about 30% efficient, and lemon-juice made out of lemons bought from Tesco costs about a penny a gram; a bit more expensive than decent non-AOC cheese. Jif lemons cost 65p and contain 55ml of lemon juice, so it's cheaper to get lemons if you've already got a lemon-squeezer; a lemon-squeezer lasts forever and costs a pound and ninepence, so repays itself over buying Jif lemons after about the first pint of lemon juice.
Tesco 'fresh-squeezed lemonade' is £1.95 for a litre, and 17% lemon, so slightly more expensive than buying the lemons; on another hand, in the 'traditional home baking ingredients' part of the store you can (according to the Web site) buy 500ml of lemon juice for 97p. It's been dehydrated and reconstituted, but I don't know to what extent that spoils lemon juice.
Juice of one lemon and 25 grams of sugar, topped up in a pint glass with strong green tea and ice cubes, is a nice cooling drink for a summer day. 15 grams of sugar isn't quite enough.