In Japanese, this is used to refer to the full work required to do a job, for example. In English, it would usually just be something like the name of that task or may be combined with a different labeling of the counting unit. It depends heavily on context.

So, instead of something like
"Air conditioner Installation
Unit/quantity (2 hours) x rate ($50) = $100"

they might write

"Air conditioner installation
set rate: $100"

or "Air conditioner installation
Standard installation fee: $100"

or sometimes even
"Air conditioner installation
Parts + Labor: $100-"

In the U.S., this notation is usually a way to get a fair profit for performing some sort of work, without having to do the fancy math to try to justify it. I suspect 一式 is used in a similar fashion in Japanese.