Check out prior comments in the Home > Plumbing link in Browse Projects, at lower left. Depends very largely on accessibility (roads, trees, etc in way), depth of burial to be below frost depth and damage from vehicles over it (typically minimum 3 feet for general protection in non-freezing areas to as much as 12 feet in cold parts of country), and how hard the ground is to dig. Also a major cost (thousands in some cases in cities or along busy highways) is traffic control and public utility hookup to their line. I have seen your size job go from as low as around $2500 and as high as $25,000 in the worst case (major street/highway excavation and traffic control and solid bedrock excavation).
I couldnot find required pipe burial depth in a quickk google search - probably 3 or 4 feet minimum, as your frost depth is rated at 1 foot, so you are goingto be towards the cheaper end with respect to trenching cost, and also in the depth range where your outage for line replacement in the same trench even with a smallish backhoe might be about 2 days rather than much longer.
For this size job you are definitely going to want 3 bids.
And consider how long you are willing to be out of water - you might consider having the new line put in parallel to the existing if you are not in rocky conditions, so it can be essentially completely installed before they cut your existing line at the house and street so you areonly out of water for a few hours. Of course, if you are in shallow bedrock, then following the old trench and putting it the same place as the existing pipe is probably cheapest and the best route - though it may mean a few days without water.
