If their costs vary, so does the bill. In soliciting for power supplies, utilities will rarely even quote a price, except perhaps in the form of a demand charge to reserve the right to certain generation capacity. The customer pays actual costs incurred plus a profit. This makes it difficult to compare suppliers, much less establish a budget. However, there is a reason for this.
Utilities receive a regulated rate of return on their investment.
This means that they are entitled to charge their rate payers for their costs, and receive a return on their investments. If costs change, prices must change to protect that regulated rate of return. A marketer, on the other hand, can give real prices. In response to a request for bids, they can deviate from the traditional cost-plus rate design and offer a dollar figure. This not only lends itself to easier comparison among marketers, but also permits the electricity purchaser to establish a budget for its electricity purchases, something which is taken for granted with other commodities.