Ideally something that's scalable. I.e. you can make it once, and sell it lots of times.

Yes and to refine that notion:

* make a $1 priced thing that 1 million people want

-or-

* make a $1 million priced thing that 1 person wants

make a thing of $1 of profit that 1 million people want

make a $1 million of profit selling a thing that 1 person wants

thanks, but yes, that was assumed, and speaking in rough terms to convey the core idea ;)