Franglais,
May I know why you want to use Write In Product instead of existing product? Because it will affect your data quality and analytic in term of data since you cannot analyze by product if it is a free text.
You are right.
If you use write in, You cannot use Price List as part of Product Catalog module for this case because your write-in product will not be assigned to specific price list and as we know CRM has markup pricing by cost or list price and stored them in the Price List in the Product Catalog.
If you don't have any strong reason to use write in instead of existing product, I agree with Fernando's answer to use override product..
Last time in a project I got markup concept.
Then I still use the existing product, use the existing price list for this product then I have 3 fields: one is price by price list (before markup), markup field, and price after markup field...
First, I was thinking to suggest you to use Price List with Markup from Costing in the Product Catalog.
You might take a look this pricing method:
missdynamicscrm.blogspot.com/.../pricing-method-in-microsoft-dynamics-crm-product-catalog.html
Since you were saying that this is will be different by Quote/Opportunity, not by Product Level, so this might not work in your case.
I think regardless it is write in or not write in product.
I can suggest you to have three fields (in the Quote Product):
1. Supplier Price (custom field)
2. Markup (custom field)
3. Price after markup (this one is the OOB field - price per unit and for customer)
This one is made from formula 1+2 = 3
So the flow, at the very first, supplier give you quote with their supplier price (a custom field will be filled), then at first of course no markup right. so the price per unit will be supplier price at that time (since Supplier Price + 0) then if you have markup, you can either key in or you have a master that store this markup....
Then you will need a javascript or plugin to calculate sum up the price to get the value to be put in the price per unit field, ok.
Make it as override (for existing product) or no for write in
I recommend you to not change the price per unit field to any other custom field because it will reflect many things, such as Extended amount (affected by price per unit * qty), discount, total line item shown in the parent Opportunity, so not recommended to overwrite this field.
Better you manipulate the value you will key in to the price per unit field.
Hope this helps.
Thanks.