There is no quicker way to make money than to develop a bunch of T3 planets that are close to one another and which produce valuable forms of spice. Getting planets to T3 can be a pain, however. This is largely because you will need three species of small, medium, and large plants; six species of herbivores; and three carnivores or omnivores.
The easiest way to get them all – and avoid repeats – is to dump all the species you are carrying, visit an existing T3 planet (like your homeworld) and collect a dozen or so of each species present. You will then have a hold full of terraforming goodness.