Space elevator dude, then your only cost is the energy to counteract gravity - which brings the second cooler idea I just had; Solar panel filled planets - cause what else would you do with all the raw material and surface area (thermal based or silicon, both could be easily setup with natural materials on an empty planet)

Edit cause I forgot to say we already have materials with the right tensile strength to theoretically hold a boulder in orbit, just not enough to get there. but we would in 300 years