The 100% eidolon completely outranks getting multiple completed shop weapons but that only applies if you don't have the eidolon in the first place. A completed shop weapon with its final break could provide a more beneficial boost than another 5% elemental boost from a single eidolon break and were talking 2 weapons? Possibly even more so. But then again, a hardcore whale probably already has a stronger completed gacha weapon grid.
However, It's not like most players are ever going to be able to buy it, 150 is simply too much for anyone that isnt a hardcore whale.
Why'd you ever get a 2nd eidolon if you could just get the eido break item for 20? Point was: Getting a 100% eido you're still missing would be a lot better than trading weapons. And ye, as you said, those whales probably have a mostly gacha grid, so it's not like drac weapons would be a big improvement.