The effort/money/time you expend will not get you the result you want. OR It will take far too much effort/money/time to get the result you want.

www.thefreedictionary.com/worth It's not worth doing. It brings no reward.

www.thefreedictionary.com/worth It's not worth doing. It brings no reward.

It might help if you remember that "It's NOT worth it" is the negative of "It IS worth it." Tom: Recently, you have been too busy to have fun with me and your other friends. Why? Mona: I'm sorry, but I don't have time for friends now. Tom: Why not? Mona: I have decided to learn French very fast. So I am taking a night class in the French language 5 times a week, twice a week I go to a movie theater that shows French-language movies, and every day I spend two hours on the phone speaking French with someone whom I met on italki. Tom: Wow! You must be very tired. You have an 8 - 4 o'clock job, and you still find time to do all those things in order to learn French. Mona: Yes, I am very tired every day. But IT IS WORTH IT. Tom: Why? Mona: Because if I learn French, I will be able to speak with that handsome French boy who has just moved into my neighborhood!

The negatives outweigh (are greater than) the positives.