There is no public transport from Murgab to the Wakhan Valley (although shared taxis should be possible between Ishkashim and Khorog). There is supposedly a not-very-regular bus that runs between Langar and Ishkashim but it’s not really helpful to many backpackers and not a viable option for independent travellers coming from Kyrgyzstan as you can’t get to Langar unless you’ve hired your own transport or got very lucky and hitched a lift. Those on a budget too tight to afford private transport might want to attempt getting into the Wakhan Valley this way from Khorog, but that will mean returning to Khorog (from where you could take a shared taxi along the M41 to Murgab)