Easy, Skylab B! A least, shunting the spare Skylab into a parking orbit once the ASTP 'milking stool' had been removed from the tracked transporter. At most the wrapping of the ASTP mission into a multi-visit set of missions to Skylab B, the Apollo CSM giving Skylab B a few boosts to keep it's orbit up. The visits would be closer to the solar maximum which bought down the original Skylab so it should have its orbit maintained for long enough.
If the Shuttle had something to Shuttle to then it would have been seen as the best thing since sliced bread.
Yes, they were even considering launching Skylab B after Challenger.
So let's say they launch the Skylab 5 flight, where the Apollo CSM boosts Skylab so that it's still in orbit in 1981. Columbia visits and pushes it higher, while resupplying it (there were many schemes to do this). This would be cheaper than the 1.5-2 Billion dollars needed for another Saturn launch.
After that, there were all sorts of plans. Using spacelabs as modules on the sides, for example. This would provide a destination for the Shuttle, so it's not just buzzing around LEO for the first decade of its life. Maybe, in addition to Shuttle-Mir, we have Soyuz (or Buran)-Skylab.
However, to make the Shuttle truly cost-effective, you'd need a mass-production of components. You'd need a dozen External Tanks rolling off the assembly line each year, and maybe an extra Orbiter to make use of them. The scale could possibly make Shuttle flights cheaper. To make the most use of the Shuttle hardware, you'd need to have two systems, sharing components.
System 1 is the STS, including the Orbiter. This would be used for Station and Satellite servicing.
System 2 is the Shuttle-derived. Shuttle-C, Shuttle-Z, or David Baker's design for an Ares rocket all fall into this category. These would be bulk flights, launching Station components, or Lunar and Mars flights.
The production of components for all these flights can drive costs down a bit.
But all this assumes a POD when the Shuttle design was finalized. As Archibald said, we could save more and make it safer by using an S-IC as the booster.