I don't see why this isn't already on the server, which leads me to believe there's either a technical issue in regards to adding it or it simply isn't feasible to add due to the time investment. Back when the servers launched, we didn't actually have a 10m cash bag and as such as this was added later to help resolve the issues you've identified.
However assuming this can't be added for whatever reason, I do fully support adding more cash bags - maybe 25m, and 50m? Making the maximum trade value up to 400m while also providing a stepping stone.
Otherwise this is a very suggestion but it's nothing that hasn't already been raised I'm afraid - maybe Nonsense has explained why this isn't on the server elsewhere? I might take a look tomorrow.