There are many factors to take into consideration when looking at the most cost effective lighting. Firstly the type of lighting required for your tank. For instance, if you have a marine tank with fish only then it wouldn’t be ideal to use Halide lighting but if you have corals in your tank then ideally you would look at using T5, halide or LED.
LEDs in general use less power than the typical Halide pendant although the initial outlay can be somewhat higher.

However with halides and T5 lighting, you have to change the tubes quite often so you will have the ongoing costs.