Because building a base on Mars is going to require quite a high amount of mass shipped to the planet's surface, much of which will be simple supplies and materials, like food, solar panels, methane generators, and so forth. These things aren't expensive on Earth - they're just very heavy.
And a base on Mars will not be a closed system. It will continuously draw in raw materials from the surrounding environment. Mars is enticing for this because carbon dioxide and water are abundant, giving you the three most important elements for fuel and food production.
