Try giving the IAC plunger a clean with TB cleaner, whilst your at it clean the butterfly and bore on the TB too.
You'll probably notice better throttle response too but the 400 idle should go when done. When I changed my TB I was really shocked at the state of the IAC for having just 48k. Backside of the TB wasn't much better either.
Atleast you can give that ago and it's good maintance before buying a new TPS.