In order to qualify as heinous a grand theft must be truly grand but there is no exact financial amount because you need to consider all the circumstances! If a starving homeless man shoplifts $15 of stuff from Walmart what they did was clearly wrong but it wasn't heinous. But if a person sees that homeless man and decides to pickpocket the entire $15 they own in the world, that's a grand theft to the homeless man and an evil act because everything they own is taken from them.

Clearly though most criminals added this way will get there because they have stolen eyewatering amounts of money or goods. If a fraudster conned Walmart out of a million dollars that would clearly count. They might also steal items of cultural, historic, or religious importance. A good measure of how heinous a theft is would be the level of greed involved. So while there is no set rule thefts should be very significant to count.