Full disclosure I'm a new DM and this is in my first campaign.

Long story short, a king of a land promised the party of two adventurers 5000 GP if they return his friend to him. The issue arises that they are currently level 2. I plan to circumvent this by having the friend of the king be dead upon finding, and if they bring his corpse to the king he will give them 500 GP instead. After I came up with this solution I realised that I have no clue what a balanced reward could and should be, how much money is TOO much money for lower levels, how much should items routinely cost? Any answers help, I can only go so far just winging it.