If you've always thought that a T-Bone steak and a Porterhouse steak were the same thing, you'd be mistaken. They're both from the same cut and they both have the characteristic t-shaped bone in the center, but this video from the folks at America's Test Kitchen explains why one costs so much more than the other.
Essentially, the real differentiator between the two is the size of the tenderloin section—or the smaller piece of meat on the short side of the bone (as opposed to the strip, which is the longer section on the other side). Because a T-Bone is allowed to have a smaller tenderloin section than a Porterhouse, that would mean that all Porterhouse steaks are T-Bones, but not all T-Bones are porterhouses. Either way, if you're ever at the butcher counter and see that the Porterhouse is much more pricey than the T-Bone (Also, if the butcher can't explain why, it's time to find a new butcher), now you'll know why, and you can opt to splurge or save accordingly.