LONDON, Jan 25 (Reuters) - Xstrata said on Friday its Las Bambas copper project in Peru would cost $5.2 billion, keeping to 2012 estimates as the miner prepares for a merger with Glencore that will see many of its growth projects come under scrutiny. Xstrata is the world's fourth-largest producer of copper and, among the diversified miners, has the largest exposure to copper, a metal that is expected to benefit from tight supply, as mine grades deteriorate and new projects lag.