“Insurance with Other Insurers: If there is other valid coverage, not with this insurer, providing benefits for the same loss on a provision-of-service basis or on an expense-incurred basis and of which this insurer has not been given written notice prior to the occurrence or commencement of loss, the only liability under any expense-incurred or service coverage of this policy shall be for such proportion of the loss as the amount which would otherwise have been payable plus the total of the like amounts under all such other valid coverages for the same loss of which this insurer had notice bears to the total like amounts under all valid coverages for such loss, and for the return of such portion of the premiums paid as shall exceed the pro rata portion for the amount so determined. For the purpose of applying this provision when other coverage is on a provision-of-service basis, the ‘like amount’ of such other coverage shall be taken as the amount which the services rendered would have cost in the absence of such coverage.”