Hi I was wondering why one was $40-$50 dollars more other than the type of battery?

The base contains electronics in it and it also comes with a shim which allows different co-witnesses.  It's an all new design as well while the 52001 uses a previously existing Holosun mount.  It does seem like $50 is a lot, I agree but everything is these days.  Just consider the SOR52102 which is another $50 on top again simply because it has a red ring around the dot!!  My advice is to make your choice based on what type of battery and/or reticle you want and then start looking for sales.