Is certified translation of diploma an absolute necessary?
Only they can answer the question as it is their rules that apply. But I think you propose a good solution. Send them a "faithful" translation that you can prepare at little cost. Offer to send a certified copy if necessary, but mention that the cost of such a translation is high. 

But you might also be able to have your university provide an English language translation or even to assure the recipient that your translation is accurate.