As somebody who lives just up the road from you (350 miles
), I think the price is fair. But keep in mind you can do this yourself. I installed DPs on my 335xi. It's a challenging job, but you will learn a lot doing so, and will make you more comfortable tackling other jobs like cleaning your intake valves.
You can find tips for installing DPs on an xi here:
It will probably take you about 5 hours the first time. Easier if you can find a buddy to help out. I'd be happy to answer questions about the install if you try and do it yourself.