Goldspan Dragon is a good card that is currently expensive due to Standard - I assume it will drop sharply after rotation. I guess it might still be relevant in Pioneer, but the competition for the '5 mana hasty dragon' slot is steep.
As for the card itself... in some ways, it only costs 3 mana due the immediate treasure production, which is its primary strength. It also makes more treasure if targeted with removal or by other miscellaneous spells. I would play a three mana 4/4 hasty flyer in most decks, but if I had a deck that cared about treasure / sacrifice, or had a deck that was playing a bunch of pump spells... in that case, it would be a strong inclusion. I suppose you could probably even do some sort of janky storm combo stuff if you had enough cantrips to target it with, given that each would generate a treasure. Alternatively, just do normal treasure stuff and stock up for a big haymaker.
....I guess all the treasure synergies running around mean that Goldspan Dragon has a lot of casual appeal / EDH applicability, so the price will probably still stay somewhat elevated even after rotation. May need an actual reprint before it drops for good.