I have that loco and the 6 cars to go with it. It's one of my favorites.
Mine is in line for a full ERR upgrade with Railsounds and GRJs smoke enhancements this fall.
Good luck with yours.
PS: To answer your question, it depends on how much you like this engine. I estimate the full ERR upgrade I'm doing will cost between $350 to $400. I don't plan on recovering that if I were ever to sell it but to me it is worth it.