You need a lot less energy to move around in space than to escape the Earth’s gravity, especially if space elevators or launch loops are built on the Moon and Mars, for resource extraction. You can travel for “free” with a lightsail, and for little more with ion propulsion, but acceleration is agonisingly slow, especially with any significant load. If it’s proved, the EM Drive, but I remain suspicious of something which contests basic physical laws (remember cold fusion?).
For meaningful thrust, you need to heat a propellant to high temperatures. A cheap way would be to focus the sun through mirrors on propellant, it will need large mirrors to get sufficiently high temperatures to get adequate thrust, but these can be quite light in weight in the vacuum of space. Towards Mars the strength of the sun tails off (about 60% of Earth values) but is more than double Earth values at Venus. Alternatively, more expensive but potentially greater thrust for large vessels, nuclear fission, there is uranium and plenty of thorium on the Moon. Disposing of nuclear waste is not a problem, compared to the intensity of cosmic radiation.
For propellant, the lighter the atoms contained, the faster they are accelerated and more thrust; hydrogen is best, water will do, but although a little less effective, carbon dioxide (stored frozen, dry ice) from Venus could be cheaper and more plentiful.
If habitats are located between Earth and Moon, then the Earth and Moon are days away, and other habitats probably hours. Mars and Venus are months away, however, and Ceres (for water) even longer. This does not mean that transporting raw materials from these locations would be prohibitive; I would imagine unmanned bulk carriers, in which case it is operating costs per ton (should be low) and depreciating the capital costs of the freighters that matter. Yet really cheap travel costs per ton on Earth is very slow – in very large oil tankers (VLCCs, around 300,000 tons deadweight) and bulk carriers (Capesizes of 150,000 dwt). They travel at around 10 knots, and a round trip from East Asia to eastern North America or Europe can take 120-150 days.
Small scale ecosystems
How much does it cost?