If  it was a firm estimate, quote, or agreement--that is, the technician agreed to fix your computer for a certain price, and there were no caveats or qualifications about the cost increasing if different or other problems were found--that agreement is enforceable. You could, if necessary (i.e. if the technician won't honor the agreement) sue him, seeking a court order requiring him to abide by the terms of the agreement and return your computer. Of course, since, unfortunately, a lawsuit is the only way to enforce your rights, it may not be economically worthwhile to do this--it may, as unfair as it is, be less expensive and more practical to pay the additional amount.