Again, the psychology is clever: you're not being asked to pay to complete one specific thing, you're given the option of using gems for a huge range of things. So it's not "I need to pay $6.98 to upgrade my Wizard Tower" (which would make most people balk), but "if I let them charge my iTunes account a bit, I get this pack of 1400 gems, which lets me skip some tedious grinding and get all sorts of cool stuff quickly".

Also, a lot of people get a single Builder early on for 500 gems ($4.99), and when you've done that once, the barrier to doing it again is lowered.