I sent a Quantum 20GB HDD in recently. They will review the HDD condition & give you a quotation. Diagnostics is free & no payment is needed if you decide not to proceed.
For a 20GB HDD, with 'cannot read' symptom. For this model, minimum is $400. If it is serious (such as head crash or scratched platter), it can go up to $2000. They commented my HDD had severe scratches because of multiple attempts to start the HDD. So if your data is critical, don't do more damage to the HDD.