Should I have the dealership change the spark plugs and valve caps for $400?

I'm guess you mean the valve cover gasket. If so, $400 is a reasonably price for both. If oil is leaking onto the spark plugs from the valve cover gasket. It would be a good ideal to replace them. The spark plugs are due for replacement, via Toyota recommendation, at 120,000 miles.

See a good independent shop!... That's too much money for a four cylinder !!!! ... (Rav'4')