Well that isn't free, most fo the cost is again paying for the HL:2 engine, but they added a bit of a cost onto the mod. I think it's actually a slightly different mod team who made it, not sure on that though. If you look at how much work went into making CSS then I'm sure you'd agree that it's well worth the extra $10-$20 that they charge for the mod it's self.
If you though HL was revolutionary keep and eye out at Value for a project called 'Portal' source engine with loads of upgrades. It will literally change the perspective of FPS. They have been experiementing with multiple spawn points in continuous motion.