I know where you're coming from, but you know how stingy WWE is with money. I'd imagine WWE wants to make this RAW huge, as they need to make a lasting impression on people to buy the network.That's a pretty soft reason not to put Brock into the chamber match imo. I doubt the difference in what he gets paid would matter a whole lot to a company as huge as WWE. Plus, Lesnar's championship win would make news and cause a ratings spike anyway. It would also be a great way to end the WWE's last traditional PPV event before every one of them (starting with Wrestlemania XXX) becomes available on the Network from here on out.
I don't think doing the title switch on Raw would be a bad move, just that it's preferable to me to do it on PPV instead.