| Proper professional letters giving bad news (as this is) should have 3 paragraphs.
Introducing the problem (i assume your costs went up, explain why/how in plain english), next paragraph, give the ammount of the increase, finally thank them for their business and appologize for the increase but the costs you have to pay to keep the equipment running went up and this is beyond your control.
Dont make one long paragraph apologizing over and over as you muddle through the detials. Make it short and to the point but always end with a sorry but we cant control when our costs go up. | 
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