You would compare it to the price of the bike when you bought it. Your actual investment price is the cost of the bike. Then you would need to determine what the price of the parts that you are selling would actually have been in relation to the bike. You can use any reasonable method to determine this cost. For example, if the original headlight assembly would have cost $200 when you bought it, then you would enter $200 as the cost or if the cost of a headlight assembly compared to the cost of the bike is 10% of the bike cost, then you would enter 10% of the bike cost as the cost basis of the headlight assembly.
You would not compare it to the rebuild price or the aftermarket price because the rebuild price of your bike, should increase due to the aftermarket price.
It would not be a reimbursement of the whole project in the eyes of the IRS, it would be based on your original cost. But in the end, if your cost basis is less than you sold it for, it will not be taxable although you would still need to include this on your return.
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