Well I thought I might save some gold actually or atleast have a use for all my farmed shards (common materials), but haven't really done the math to be honest.

From what I have gathered: base + base = rare dual armor (atleast for some combinations).

15 base + 15 base = 15(?) rare dual -> Unlocks + version and you still have a decent armor (and a good one to use for more fusing). 25000+(3900+600+1200?+1800?) ~ 32500g

Unlocking + by enhancing requires getting the armor and enhancing with quite a few uncommon armors. Not sure how much it costs to level a rare to lvl15 this way, but it sure takes a lot more uncommon materials.