At about $20.00 per 10' 3/4" ($2.00/ft), may be the most expensive option. Depending on soil conditions, drainage, and bedding material used around the pipe, copper can corrode.
CPVC (Chlorinated Poly Vinyl Chloride)
3/4" schedule 80 CPVC will cost about $15.00 per 10' ($1.50/ft), slightly cheaper than copper and can be easier to work with than copper. CPVC is a hardy material, and should have no problems in any soil type.
Plastic Pipe and Fitting Association FAQ (CPVC).
HDPE (High Density Polyethylene)
3/4" HDPE will cost about $45.00 for 100' ($0.45/ft), so depending on the amount required can be much cheaper than the previous options. HDPE is used by many municipalities as water supply lines, so it has been tried and tested in underground applications.
Plastic Pipe and Fitting Association FAQ (HDPE).
PEX (Cross-linked Polyethylene)
3/4" PEX runs about $40 for 100' roll ($0.40/ft), so again depending on the amount needed it can save some money. There are mixed reports about using PEX underground. Some people suggest installing the PEX in a conduit (PVC/CPVC) to protect it (which raises the cost), while others report it can be directly buried without a problem.
Plastic Pipe and Fitting Association FAQ (PEX).
In all cases you'll want to make sure the pipe is buried below the frost line, and follow proper procedures for burying the specific material (depth, bedding, etc).
Both Copper and CPVC will require more fittings (depending on run length) than HDPE or PEX, but for short runs they may actually be cheaper to use. PEX and HDPE both come in rolls and are flexible, so they can be easier to work with than a rigid material. Depending on the run length HDPE seems to be a cheap, easily installed, reliable choice.
Make sure you check with your local government for rules and regulations before making the final decision, and always Call before you dig.
Estimated prices provided by Google Product Search, at the time of posting. Prices may vary over time, and by location.