The UKTram website has a report on costs of tramways at
This should be the starting point for any discussion of tram costs.
As Mat says, the cost can depend on all sorts of factors, but in general £20-£30 m per km is a good ball-park figure. This would include the cost of vehicles, as with a typical service frequency the number of vehicles is roughly proportional to the track length.
Costs should include the cost of utility diversions and obtaining TWAO. It might be possible to economise on these, but you are getting into experimental territory here.
(David Walmsley – Transport Analyst BSc – PhD, CMILT, MCIHT, tram technical lead. Member of UKTram; UK Rep on European Urban Tram Forum; formerly Fixed Track Executive at Confederation of Passenger Transport)