What's the accepted way to add different prices to commerce products?
As mentioned you can add new field to your product type:

Next enable commerce_product_pricing_ui module or just add new Rule on event “Calculating the sell price of a product”. Rule looks like this (full size):

You can control the order of Product Pricing rules by Rules weight.  

But I prefer to create field “Old price” (or “Common price”) and to add simple description:


  If you want to create special offer for this product, please, place new price in the Price field and move crossed out price to “Old price” field. 


So, “Old price” is only for theming. It works for small commerce sites and I don't need any crazy Rules.