How much would it cost to get from Xian railway north to the big wild goose pagoda?

I think you mean the North Railway Station, where high speed trains arrive at, right? If so, you can take the subway line 2 from the North Railway Station to Xiaozhai Station (CNY4). Take the Exit D and walk eastwards along the East Xiaozhai Road about 0.9 miles to the north square of the pagoda.
If you don't want to walk, you can take bus no.400 or 34 to reach the destination.
If you want to take a taxi to cover this walking distance, you'd better get off the subway one stop earlier at Provincial Stadium, as it is difficult to hail a taxi at Xiaozhai. And the fare would be CNY10-12.

If taking a taxi the whole way, the fare would be CNY70 – 80.