If you don't work much with the Product and Price List functionality inside CRM you might not have realized you can't simply enter a negative amount in a Price List Item. Why would you do this in the first place? You might have a situation where a customer sells a base unit that comes with a standard set of options, along with that there can be additional, enhanced options that can be purchased that would increase the overall price, but there might also be lesser options that reduce the overall price.
A quick way around the limitation of not being able to enter a negative amount is to use a Discount List.
Create the Discount List from Settings -> Product Catalog -> Discount Lists
Assign a Name and set the type to Percentage.
Add a new Discount. Since this isn't really based on the quantity purchased, set the Begin Quantity to 1 and the End Quantity to a large number. The key here is to set the Percentage to 200 percent.
For each Product price you wish to make negative, assign the Discount List to the Price List Item.
Now when we add the Product to a Quote, Order, etc... and Save - you should see a negative price.