And the space station would do what exactly, pull materials out of a magician's hat? If all the raw materials are eventually going to come from earth anyway, you're just adding intermediary steps to increase costs. Here's a number of reasons why you might want to have a space station, but practically doesn't apply today:
1) We can gather and refine materials and produce parts/fuel with the required tolerances/quality from a lower/zero-g gravity well like the moon or asteroids at a lower cost than shipping it from earth. For example say you discover an asteroid full of bauxite ore. You still have to create a mining ship, send it to the asteroid belt, extract the bauxite through mining, ship it to the space station, capture it, smelt it, roll it to sheet metal and cut it with extreme precision. While launching it from earth is expensive both the setup costs and production costs of doing it in space dwarfs the savings.
2) You can likewise gather energy by for example setting up gigantic solar panels that can charge a space craft. However, none of our current rockets run on electricity and the energy costs of orbiting/deorbiting a space ship probably dwarf the energy gains.
3) You can refuel/repair/retrofit/repurpose incoming spaceships without going down into the gravity well, however we generally don't do return missions because of the costs involved in sending them back to earth. It may be useful if we have big reusable "ferry" ships between say Earth orbit and Mars orbit, though even then it's questionable if we shouldn't just launch resupply rockets directly rather than going through a space station.
4) We could build ships that aren't hampered by the launch forces and are designed for zero-g only, but practically we're pretty good at various forms of fold-out designs that'll survive launch and transform into a more fragile shape in space. Same with the rovers, they hit the ground curled up like a ball then deploy.
5) We could build bigger ships than could be launched through a single launch, however the total launch costs would be the same. Practically we'd probably build it like the ISS though through interlocking modules, with large interlocking sections it should be almost as structurally sound as building it in one single piece in space.
6) If we have a lot of cargo going between many different systems then a hub-and-spoke system is more efficient than direct peering. In the foreseeable future though, everything will either come from Earth or go to Earth so this isn't relevant until we've got major off-world colonies.
The TL;DR version: Nothing a space station could do would lower space exploration costs today, only increase them.