Depending on where you fly, you can actually save money by doing your CFII in a twin. Lot's of places have a PIC requirement before you can instruct. The training for the CFII would count towards that.
I'll do a little math. Let's say it takes 10 hours of flying to get your CFII. At 150 bucks an hour in a twin, that's 1500 dollars. If you do it in a single it would be 75 bucks an hour totalling 750 dollars. But if you do it in the single you still need the PIC in the twin so you can instruct. So add the 750 to 1500 and it ends up costing more.