While I agree the PC solution may be more flexible, I don't think it would be less expensive.
I started down this path after I wasn't able to grab a used Sega Ferrari F355 deluxe 3-screen setup. This setup is a lot more flexible than if I would've grabbed that arcade, but obviously not as much as if it were a PC. I still think the Sega would've been cool, I keep telling myself I got a more mature, update-able, more tracks, more cars, etc. I never really got into Gran Turismo, so the fact that it is entirely new to me is also appealing.
Cost-wise, I'd be curious to see the math. The PS3s are priced well on the used market now. They were maybe $100 each. Technically, I already owned 1 ps3, but lets call it $350 if I built it from scratch today. Do you think I could build a PC for $350 that will drive 3 32" screens? Maybe I'll hook both up! I think each TV has a VGA as well. I'll have to check.