This is oversimplifying things a bit.
It can categorically be stated that for a year's worth of CPU compute, local will always be less than Amazon. Of course, putting percentages on it doesn't work - there are just too many variables.
There are many admins out there who have no idea what an Alpha is who'll swear that if you're not buying Dell or HP hardware at a premium with expensive support contracts, you're doing things wrong and you're not a real admin. Visit Reddit's /r/sysadmin if you want to see the kind of people I'm talking about.
The point is that if people insist on the most expensive, least efficient type of servers such as Dell Xeons with ridiculous service contracts, the savings over Amazon won't be large.
It's a cumulative problem, because trying to cool and house less efficient hardware requires more power and that hardware ultimately has less tolerance for non-datacenter cooling.
Rethink things. You can have AMD Threadripper / EPYC systems in larger rooms that require less overall cooling, that have better temperature tolerance, that're more reliable in aggregate, which cost less and for which you can easily keep around spare parts which would give better turnaround and availability than support contracts from Dell / HP. Suddenly your compute costs are halved, because of pricing, efficiency, overall power, real estate considerations...
So percentages don't work, but the bottom line is that when you're doing lots of compute, over time it's always cheaper locally, even if you do things the "traditional" expensive and inefficient way, so arguing percentages with so many variables doesn't make any sense - it's still cheaper, no matter what.