I asked a similar question about a month ago... I know the search function isn't working, and it was 3-4 pages back.
When do I start buying tech in tech trades?
From what I gathered, the 4K infra jump appears to be the most ideal. At that point, the money you save on bills due to more tech is greater than the profit you would receive from selling tech.
The best way to 'graduate' is to start at 0 tech then buy 5 batches of 50 tech and send them to trusted nations for them to hold on to. Then buy another 50 tech for yourself. When you're ready to graduate, ask for all 250 tech back at once. When it's all said and done, you will start off buying tech, with 300 tech in your pocket, and you paid 6*50 tech as opposed to the increasing price of tech up to 300 tech.*
*Kudos to Blacky on this advice