You can set mandatory dimension, but this is not related with unit price. This is related with control of using dimension value. For unit price, the best solution is customization as Mohana said.
But, sometimes 0,00 in unit price on sales line is must to have. This is the way if you need to ship present to your customer e.g. I will be thining once time about this. Maybe using item price list is better solution, because you can get default unit price,