The compensating advantages which have since accrued to us were not available to them. It we are talking about cost in terms of energy expended, the answer is no. He trained both animals to run on a treadmill while their oxygen consumption was measured. Both primates expended the same amount of energy whether running on two or on four legs. On the other hand, bipedal locomotion WAS an option available to protohominids.
Most primates exhibit a variety of positional and locomotor behaviors.
Humans are hominoid primates, and thus come from an evolutionary background in which bipedal positional behaviors are common. There are numerous examples of particular positional behaviors that are inefficient. Most current models for the origin of bipedalism posit frequencies of bipedal behavior that increase through evolutionary time. At any particular point in time, the organism's overall adaptive strategy had to be efficient, however it's bipedalism did not. Increased frequency of bipedal behavior would have caused selection to favor morphological configuations that make bipedal locomotion more efficient.