The income rate on a starbase is a multiplier of your income-per-trade-port based on how long your trade chain is. So if you're getting a trade rate of 2.0 per trade port, the first level as TEC gets you 2.0 x 1.3 = 2.6 cred/sec. If you upgrade it to 2nd level as TEC then you get a 3x multiplier which is 2.0 x 3 = 6.0 cred/sec.
These upgrades are cumulative with planetary bonuses, trade pact, arbitrate tariff, and TEC civ upgrades. A well placed TEC starbase can earn upwards of 25 cred/sec in the later stages of the game.
