One of my players is playing a Blood Hunter from the D&D beyond site. He wants to have his character be the Ghost Hunter based subclass and have his backstory be that he grew up in a ghost hunting family/group, who all worshiped or looked to one or two gods. What mechanically is allowed to be a holy symbol?
The rules text on holy symbols is pretty brief:
A holy symbol is a representation of a god or pantheon. A cleric or paladin can use a holy symbol as a spellcasting focus, as described in the Spellcasting section. To use the symbol in this way, the caster must hold it in hand, wear it visibly, or bear it on a shield.
There are a few different options listed in the rules - amulet, emblem, and reliquary - but they only differ in weight (1, 0, and 2 pounds respectively). They all have the same cost. Exactly what they are (beyond the regular English definition of the words) is not defined - it could be steel, gold, stone, etc. As long as it's worth 5 GP and has the appropriate symbology, it's a holy symbol.
The nature of a holy symbol is not a matter for the rules, because there are no mechanics behind it. Even within the same church, followers of a given deity may use different holy symbols. The themes and design of the symbols follow a pattern (in the Realms, Kelemvor uses a scale) but the exact construction varies.
In fact, the price is only an issue for buying something that qualifies for the game mechanical use of a holy symbol, which is only relevant to Clerics and Paladins. For a lay worshipper, a token of bone or wood with the right markings may just be good enough.
Without a mechanical requirement, there is no limitation
The Blood Hunter class does not use a Holy Symbol, so there is no mechanical requirement that needs to be resolved here.
Because of that, you are only limited by what your DM decides is a holy symbol in their world. You'll need to ask your DM, or if you're the DM, you'll need to do a bit of world building.