You are not entitled to a larger refund than you paid for under the law of germany.
Your contract was for Part A at the Price of X. They have to either fix it or get you back price X. They can't get you part A, so they refund you X.
§437 BGB stipulates your right to have either the item fixed or replaced ("Nacherfüllung/Nachbesserung" according to §439 BGB). In the alternative, the price for the defective part could be reduced. Alternatively, you can have the contract voided in its entirety (return of parts and money to original owners in exactly the same amounts). Atop that comes the possibility to have fees (e.g. postage or inspection fees) and other "Schadensersatz" repaid that the customer incurred. §438 BGB specifies the timing requirements.
Do note, that Nacherfüllung under §439 BGB (4) is limited: If the costs and fees would be too high, the seller can deny it. So if you were promised to have a working car engine for 1€ and it doesn't work, you can not force them to buy you a new one for thousands. So, Nacherfüllung can force a vendor to swallow some losses, but not infinitely so.
If since your purchase the price for part A has risen, you are not entitled to be repaid the new price - just like they can't just refund you less because the price for the part crashed. If you insist on having the part fixed or replaced, it is the seller's obligation to attempt to do so, but if they are not able to do so (e.g. the part is no longer made and no stock available), the only economically viable option is to annul the contract.