English idiom for 'Fry the fish using fish's own oil'

You might want a to use an actual, well-known economics term:

• R.O.I: "Return On Investment"

Or

• "Pays for itself"

You would then explain how the subject does this.

Or you might want

• "you can't make an omlette without breaking a few eggs"

suggesting that a price MUST be paid to get the intended benefit.

or

• "Killer app", short for "killer application"

which refers to the fact that the very first spreadsheet program for a personal computer allowed accountants|book keepers to justify the entire cost of purchasing an entire expensive PC system because it would divide their workload by a factor of 10 or more. If the computer ONLY ran a spreadsheet, it was worth the price to them. But a computer also does much, much more, like print layout for documents, art, e-Mail, etc..

You might want:

• "Get the ball rolling"

Which conjures the image of giving a boulder a tiny initial push to start it rolling down a hill, after which gravity will take over & keep it rolling.

After (as rileywhite says) you have 'primed the pump', the enterprise will hopefully carry on under its own steam.

But avoid mixed metaphors even more than 'hopefully' as a pragmatic marker.