Something substantial is large in size, number, or amount: If you want to say someone spent a lot of money without being too specific, you could say they spent a substantial amount of money.
The adjective substantial descends derives from the Latin substantia "substance," which means "stuff." A substantial meal is large enough to satisfy hunger. A substantial structure is strongly made or built. And a substantial man possesses wealth and property. Related to this last sense, "a man of substance" refers to a wealthy man with property.