By default, as you add any item to a proposal, the sell price will be the product's MSRP. You can manually set a specific sell price on an item in Portal. Additionally, you can change the default sell price to be a market up from cost.
Default Sell Price Setting
To view and set your default sell price for all catalog items, go to your Settings in Portal (Click "More" on the left navigation menu). Scroll down to the Catalog section, and you'll see the Default Sell Price setting:
Custom Sell Prices
When adding an item from the Catalog to your proposal, if you click to edit the price, you'll be asked if you'd like to make the price your Company Default Sell Price for that item:
Alternatively, from the Item page for any item in the catalog, you can click the Sell Price to edit:
Please note that a custom sell price will override the Default Sell Price setting, and any changes to cost or MSRP will not change the Default Sell Price once it is set to custom.