Tax cheating is a crime. Not really ambiguous or subjective. What is subjective is how sales take place and what true cost was.
You can (try your luck) claim the cost of storage and transportation in addition to purchase price. You can claim the highest customary and allowable charges. No law says you did not pay $10 per coin, per year for safe storage. Be ready to prove it if asked. You also bought expensive insurance and paid for secure transportation all adding about $300 per coin to original purchase. All legal. You can claim you paid a 15% commission upon purchase and again on sale. Be ready to have proof.
But make no mistake tax avoidance is a crime.