Enchanted gold apples offer a couple more buffs besides regen. Apparently you also get Fire Resistance and normal Resistance for 5 mins. And its regen effect is more potent.

All that gold... could've been 72 power rails

No, it doesn't cost xp. Just a normal apple on a crafting bench surrounded by *sigh* 8 gold blocks *sigh*

Actually you don't need eight gold blocks because the reason nuggets were made for the same reason, so what I'm saying is that you can put a gold ingot in the crafting table and make nuggets. Then take those nuggets and put around the apple to make a golden apple (I made like 7). Though it is a golden apple it doesn't give you the same effects as one with gold blocks, so it's weaker. Hope this helps.