User blog comment:Luxionmk2/Researching Efficient Fusing/@comment-68.1.33.56-20140304231241/@comment-5007614-20140305001624

My experiment with the Metal King yielded a 40% loss of XP. So if that is consistent, then theoretically, five Metal Gods fused in the same fashion would yield a unit that would provide an extra 150K, for a total of like 152K. Note that this is strictly for a unit of a matching element.

Let's just look at 150K. The cost to fuse that giant slime ball, into a Level 50 unit for example, would be 60,000 Zel, not including the costs to create it. That's 2.5 XP/Zel, which is insanely good. To be consistent with the rest of the article, that's 0.4 Zel/XP, and lower is more efficient.

Fusing those 5 Metal Gods direct will give you over 251K XP, but cost you 255K Zel. That's almost 1 XP per 1 Zel, which is still good, but that's because these are Metal units.

Now unless you have Metal Gods coming out of your ears, I'd rather just spend the extra 200,000 Zel and fuse the Metal Gods direct for the extra 100,000 XP. Easier to spend a single weekend recouping the Zel in the Golden Vault rather than doing a hundreds of Metal Parades.