Worth is something only YOU can determine.
$30 is certainly cheap enough to try the experiment.
You will need something like a 800w psu to handle the power requirements.http://www.realhardtechx.com/index_archivos/Page362.htm
If your interest is in FPS benchmarks, CF is certainly attractive.
But, your gaming experience may not be so good.
dual gpu is prone to screen tearing, stuttering, and non support in many games.
With the advent of strong single cards, and more to come, the use of sli/cf is being de-emphasized.
I think I would point towards an upgrade like a GTX1060 or RX 580.