Professional landscape LED low voltage lighting made by manufacturers like Kichler, FX Luminaire, and Volt Lighting will generally cost you between $250 and $450 per fixture, installed. This typically includes the labor, lighting cable installation, the transformer and the fixtures themselves.
Other manufacturers may offer less expensive options, and/or, your local contractor may have some alternate options to present you. There a lot of companies out there producing lights, and LED quality varies between manufacturers. If a warranty is offered, that is typically a good sign and highly recommended in case something happens to your lights.
If the price seems is too good to be true, it probably is!