Yes the location is near the current junction for the siding to the Boral Deer Park West Quarry. This is also where the dual track becomes single track, and where a new road overpass has recently opened for the new extension to Christies Rd which provides access to said Quarry (the old access road was closed for the RRL).
As for the inflated cost, two things cause this:
The first is the Disability Discrimination Act (DDA)
The second is the no new rail level crossings policy the government has adopted (which includes petestrian crossings).
The two of these policies combined heavily inflate the cost of most modern railway stations. That is unless the station is only of a single platform, and at the same time only needs to be accessed from one side of the track (like Talbot for instance). If not, a footbridge is required, which also includes the need for both a set of stairs for each access point (each platform and each outside station entrance) as well as a DDA compliant ramp for each access point (these ramps can easily be almost the length of the platform). If you want to add the costs of lifts for each of these access points (as is the current trend) that will also add a fair bit to the cost.
All of this and we still have upgraded signalling to consider, the possibilty of a new track layout (if they are building it as a 2 faced island platform this is a necessity), large sealed carpark (which will no doubt be full by 7am on most workdays), bus interchange, CCTV security and PSO offices and all of a sudden it's not that difficult to see how the costs begin to skyrocket.