It costs $65-per-hour to spy on your ex-husband ... this according to a private eye who claims he spent months snooping around for "Gossip Girl" star Kelly Rutherford ... only to get stiffed when the bill came.
The divorce between Rutherford and her ex-husband Daniel Giersch was EPICALLY NASTY -- with restraining orders and allegations of child endangerment.
According to a lawsuit, obtained by TMZ, Rutherford hired a private eye to spy on her ex in April 2010 ... "in relation to certain issues regarding child custody and other matters."
The private eye, Edward Banach, claims he worked on the case for a couple of months ... racking up a total bill of $86,249.85 ... but only got paid $28,214.35.
Now, Banach -- who was sub-contracted through a now-defunct security company -- is suing the business AND Rutherford to get the remaining $58k.
FYI -- At the height of the legal war between Kelly and Giersch, the actress admitted she hired a P.I. ... to prove her ex didn't properly secure the pool and play areas at his home -- which she said endangered the couple's 2-year-old son Hermes.
Rutherford explained, "I was concerned about my son's safety."
We called Kelly for comment on this story -- so far, no response.