To prevent historical price data being changed, disallow editing of product prices that have a From Date < today.
Editing of future dated Unit Prices will still be permitted.
To simplify creation of new unit prices, adding a new Unit Price should:
copy the Cost, Markup, Price and Max Discount from the prior Unit Price, if any
set the From Date to the current date, or the previous price's to-date, if there is one
There will be no restriction on administrator users from changing prices.