It's not the wafer cost. The masks (the "negatives" for the lithography) are the problem. A mask set (you need multiple exposures for one device) for a modern EUV node costs 20-30m$. That's the limiting factor. You can't get cheaper than that.

As a sibling comment notes, multi-product wafers are a theoretical answer. However, since you have process corners (manufacturing defects aren't uniformly distributed on the wafer), it is unfeasible for anything but the cheapest parts.

The real next moonshot in the foundry business would be to lower the respin costs, i.e., the amount of money it costs when your fabbed first silicon doesn't yield or validate functionally in the way you had expected/planned.

If I were the US government (or any other), I'd focus on that. Subsidize the respin cost to zero in the short-term, given certain prerequisites for start-ups, and push an all-out Manhatten project RnD effort to lower the respin cost in the long run.