Lemon extract is not the same as lemon juice; it is far more concentrated. The flavour of what you are making won’t taste the same as when lemon zest or juice are used.
There is also Imitation Lemon Extract, which is cheaper.
To make your own, bring just to the boil peel of 1 lemon in 1/2 cup (4 oz / 125 ml) of vodka (make sure there is no bitter white pith on the peel). Remove from the heat when it reaches a boil, then put into a jar with a lid (peel and all), screw lid on, and let infuse for at least two weeks before starting to use.
Other extracts such as vanilla or orange, or lemon zest
1 teaspoon lemon extract = 1 teaspoon zest
Store away from heat and light at room temperature for up to 1 year.