Options:

• LEED rated toilets
• Low-flow showerheads
• Aerators on taps
• Grey water recycling
  • Re-use water from sinks / showers
  • Requires double-piped drainage throughout the home
  • - Not a great return on investment

Conclusion: Insulate any hot water feeds.

Cost: $14,400 = $6 / sqft * 1,800 sqft

References:

• Ecohome plumbing
• How to estimate plumbing costs