In this answer I'm assuming that the basic circuit is a flyback converter that regulates to produce a 5 volt DC output. A regular old-fashioned power transformer cannot be relied upon to produce a stable 5 volt output due to the transformer's input AC voltage changing and because of its inability to provide decent regulation against load current changes.
The cheaper option is to have an extra winding, rectifier and smoothing capacitor but only by a few cents I suspect. You have to offset the extra few cents you might pay for a buck converter with the extra performance in terms of regulation you get from it when sharing one winding.
This is because only one transformer winding (in a flyback design presumably) can be regulated and the others follow suite. You might find that a heavy load on the 5 volts will increase the DC voltage on the rectified outputs of other windings and this may be a step too far - of course with one winding on the flyback transformer you can use the PWM to regulate it to 5 volts and, if you tee-off to another regulator there's no worries about that producing a stable output because it is a regulator.
It's unlikely that an extra buck converter on the 5 volt DC output from a flyback circuit will cause much of a problem but, I'm generalizing and the devil is in the detail so I can't guarantee this.