cirtcele,
I am in the utility industry in the South and what we are seeing right now is transformer prices escalating all the time and prices varying wildly between manufacturers, probably due to core steel and copper prices. You need a tight specification and I would suggest (as we do) bid each order.
Short answer to your question is that I don't think there is a base estimating formula such as "$XX per MVA". Much of our estimating is done by recent quote and ordering history.
I believe some manufacturers will guarantee the price for a certain tiime period based on their copper and steel stock at the time. Sometimes there will be a built in escalation clause.
We just bid a 5MVA 26.4/13.2kV "step" transformer and the prices ranged from between 160K and 270K. A huge range in prices.
One friendly note, read and compare your your quotes carefully.