Half the price of github is not great right now, this space is heating up! Ubicloud is 10x cheaper and https://runs-on.com is in the same ballpark by using spot instances. (Currently switching to RunsOn)
But besides just compute, I think the bigger long-term unlock for build performance is a new distributed compute engine, to free build workloads from single machines. We've started building this for our container build product, and plan to integrate Actions jobs as an input as well, starting with the cache integration we have today.