By default, as you add any item to a proposal, the sell price will be the product's MSRP. If you change the sell price from the product details page in the catalog, Portal saves that price as your default sell price for the next time you add that item to a proposal:
However, if you add the item to a proposal and then manually change the price from the proposal page, it will only apply to that project and won't save for future use. The same is true if you change the price on the modal window (where you choose which proposal to which you'd like to add the part).
There are a few other options to determine your sell price by other factors. In your Settings page, under the Proposal section, you have the option to utilize a default markup % for all items you add to proposals, instead of the most recent sell price. Once you edit a sell price, however, it changes the system back to Last Sell Price.