To prevent historical price data being changed, disallow editing of product prices that have a From Date < now.
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
set the prior price To Date to the From Date. If the From Date is changed, the prior price To Date should be updated to reflect this.