The palce where the you get the most gain from saving weight is the outer edge of rim. So if you spend a little money to use light weight liners, I think it is worth it. The extra miles per hour you get over the long run vs. money spent is a no brainer.
If you use three layers of duct tape you will add a lot of weight right where you do not want it. You will be noticably busting a$$ more and getting to where you want to get in the same amount of time.