The average nation will start making as much as he was before after bills at ~1050 infra, however unless you buy a shitload of infra, your growth will be stunted due to the massive increase in cost per level of infra.
I recommend saving 2 mil, but I have done the jump twice, both times with 1.2-1.5 mil, and it didn't hurt.
edit: both times I jumped, I had 5 factories as well. If you don't have facks, go for at least 2 mil.