700 diamonds, you will find, is not much. I think I'd combine the advice above and try to increase my diamonds faster than the rate of increase for the magic residences as you go up each chapter. If that sounds confusing, it is, even to me, who wrote it. The cost of magic residences goes up and thus, they are cheaper in lower chapters...as
@Darielle and other state. But it also costs a blue print to move it up a level AND you tend to get more diamonds as you go. So if, say, the next chapter that 500 diamond magic residence costs you 600 diamonds, but you have, in the mean time, both collected a blue print or two AND more than 100 diamonds, you are ahead by waiting for the next sale. As long as you are earning more diamonds each chapter than the increase in the cost of a magic residence, you will eventually have enough blue prints saved and diamonds to buy it at that current chapter. But of course, in waiting you do lose the benefits of not having it...more pop and coins... but in the lower chapters it's not that much. I think somebody said about chapter 8 or so would be a good compromise of when to buy them? I think that sounds about right to me.
AJ