Ahh, I did not know that. Still though, tis £13 more expensive just for some weights you can shove in the bottom. Although I do like that electric blue skin...
I don't know that the £13 extra is just because of the weight, it also includes the ability to adjust DPI on the fly as well as adjusting the report rate (max of 1000, I think the report rate on the MX518 is probably 500) as well as a higher top DPI which may be a very good thing for some people... I personally have mine on 800DPI