Heating - Oxygen Barrier PEX Pipe
http://www.supplyhouse.com/Bluefin-T050-1000-OXY-1-2-Oxygen-Barrier-PEX-Tubing-1000-ft-Coil $237.95 for 1000 feet 1/2". 23.8 cents per foot.
For radiant heating it is typical to use 1/2" PEX.
Note that Oxygen Barrier PEX is less expensive than Non-OB. Non-OB is 35.6 cents per foot. This is because OB PEX does not have the expense of being certified for potable water. A building inspector will not allow it to be used for domestic hot water.
In practice it is the same as regular PEX but with an oxygen protection layer added to the outside.