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

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.

You may be looking for something along the lines of "prime the pump", meaning that you put some intial investment into something to get it started, and then it can continue on its own. This is often used in reference to Keynesian economics, but it can be applied more generally, as well.

Literally, it is an expression referring to adding water into a water pump so that water can be extracted from, for example, a well. See this instructional guide.

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.

An approaching idiom is "Wilful waste makes woeful want" [willful in USA], meaning that if you don't spare enough you are going to ruin. The idea is similar but expressed in the reversed way.