Buy the first two Civilian Infrastructure upgrades, that will negate the upkeep costs for the planet immediately, as well as raise the population cap to a comfortable 70-100 (except asteroids, which are capped at one). Between removing the upkeep and taxes from a growing population, you'll recoup the investment fairly quickly (about 5-8 minutes of game time, depending on loyalty). Trade is important later in the game when your empire becomes more far-flung. Advent, who get culture techs early, defer trade, but lose less income to disloyalty. TEC get trade fast, so they can quickly get a thriving trade route going, but building out a trade route is much more costly than planting an early transmitter. Vasari have late access to both loyalty and trade, but their scouts let them dominate neutral resource nodes, which they can sell on the black market to supplement their early credits.
But in all cases, expansion is the lifeblood of your early economic development. Even if you sink a bunch of credits into infrastructure, remember that you're gaining the resource asteroids of the system, and will use those to defray the costs of your expansion, and later, the size of your empire will ensure a robust and efficient trade route. When building your trade route, make sure you don't close a loop that might make it shorter.
Refineries are best used, IMO, to leverage well-placed planets, especially one that is off your trade route, yet adjacent to a lot of asteroid-rich systems.
PS: Just remembered: Don't forget, every planet you conlonize has a certain number of free logistical slots, and the first upgrade is the cheapest. So that tiny asteroid may have cost you 450 credits and 150 minerals to buy off its upkeep penalty, but you get the free logistical capacity with it. So, expand, expand, expand.
