Many thanks for your help!!

Assuming that this question is related to odoo version 10. 

You have 2 choices to manage cost price

1) Deactivate the Variant on products
to do that go to Sales-->Configuration-->Settings and choose option 'No variants on products'.  Then Apply
Now the system will allow you to input cost price on products.

2) Manage Cost price on Product Variants. 
You can access product variants from Sales-->Sales-->Product Variants

Hope this will help  you.

Cuando creas un producto tienes abajo a la derecha dos campos para llenar: Precio de venta y Coste, precio de venta es el que se mostrará cuando hagas un pedido de venta y selecciones el producto, este se llenará automáticamente-

When you create a product, you've got a couple fields to fill, "Sale price" and "Cost" if you fill the sale price field, when your making a quotation you'll be able to see that when you select a product, the price you defined will be automatically added to the field Price or something like that

I hope this helps!