Over the years, there has been at least a couple good proposals to develop a cargo-only booster derived from Shuttle hardware. Back in the 1980s, there was the Shuttle C proposal from Marshall Space Flight Center. It would've used the SRBs, the ET, and 2-3 SSMEs with a cargo pod capable of putting somewhere around 45 to 77 metric tons into LEO. The SSMEs would've been recoverable, lowering the costs. It claimed a fly away unit cost of $85 million, a number I find very hard to believe.

Later, Zubrin proposed the more powerful Ares HLV. It would've used 2 Advanced Solid Rocket Motors (canceled), a modified ET, and 3 SSMEs to put 121 metric tons into LEO. No mention on the price, but since the ASRMs were canceled, you'd either have to restart that development program or accept less payload to orbit.

Of course, it might be more cost effective to buy or produce under license the Energia HLV. The RD-170 engines are still in production (and we've negotiated to build the RD-180 derivative engines under license for the Atlas program) and the Zenit strap-ons are still in production (and Sea Launch has experience with them). We'd need to build the central tanks (shouldn't be much harder than building Shuttle ETs), avionics, and payload pods, but that should all be possible. The baseline Energia was rated at 88 metric tons to LEO with a lot of growth potential and payload flexibility. We'd need to set up a new launch pad at the Cape. Still, that might be less expensive than developing a new American HLV program, even from Shuttle hardware.