A leak-down test is basically hooking up compressed air to the cylinder and seeing how fast the pressure leaks off. It's unlike a compression test (that uses the starter to crank the motor and reads cylinder pressures) because it's done w/o having to turn the motor.
Here's a decent explaination:
http://www.type2.com/library/engineg/leaktst.htm
I have no idea what they charge... I'm assuming it's their normal $75/hr shop work rate, and probably an hour for all four cylinders. On an inline 4 it would probably take less time since the cylinders/sparkplugs are far more accessible.