Since most of the hop ups that people buy are in the areas of power (engine), suspension (shocks), and strength (towers, arms, bulks, etc.) a kit would be a great place to start, BUT it would not be cheaper in the long run.
I picked up an RTR and the hop ups I wanted on the same day. I removed the unwanted parts and sold them. So far my Savage with hop-ups has cost me about a grand. Going the kit route might run you about the same.