throw good money after bad
Waste more money in hopes of recouping previous losses, as in Hiring him to improve that software is throwing good money after bad; it's based on an older operating system and will soon be obsolete. [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.