They aren't that much heavier. The main problem with Heims is wear over time and the fact that they get chatty once they do. The JJ is basically a poly bushing around a Heim joint, so it is a little bit softer and it's not as noisy (not metal on metal).
If your just looking for longer joints, you can just stick with rubber on both ends. But you won't gain much flex. You can also just weld large bolts onto the back side of one end and put a bung on some tube, you will gain some flex at lower speeds as the tube will be able to rotate. Then you just have to worry about your weld shearing off.
There is an older post on a Tracker that did this, looked great and supposedly worked great as well.
A fully forged JJ with 3/4" thread is only $40,
http://www.currieenterprises.com/cestore/Product.aspx?id=1262 and a good set of heims are going to cost you around 20-30 per joint so your not saving a lot. If you want to re-do your rear suspension expect to spend at least 300 in tubing, joints and bungs. If your looking to build a full long arm kit, especially for a DD, I would plan on spending the money up front for solid hardware no matter what links you decide to use.
Or you can just go with the middle paragraph and run it until it breaks.