Just skip it for 2-3 gems I forget which it was I believe 2 but not positive, anyways thats what I did to get past it, there was no way I was spending 12 gems on this thing that I would not use. =x
I, as well dont think its right that a goal involves purchasing something for gems. I had a similar problem in Vday edition RS, it required purchasing 4 tables 3gems each, well 12 gems later and there was NO reward at the end of the quest line, I was pretty upset. And like this quest you are referring too, I dont believe the reward was worth even skipping it for 2-3 gems.