Any idea where that $50,000 number came from?
Used modern trains, especially from a one-off gauge like G, go for a fraction of their estimated value.
Best way to realistically gauge value (pun intended)
is to search on ebay for similar models and see what the selling prices are. Don't fall into the trap of going off of asking prices.
Another option is to register at trainzauctions.com. They have a site where you can upload your inventory and a paid membership feeds in real time values into your collection to estimate worth.