Pricelist Price Based on Custom Value

This module extends the product pricelist so that pricing rules can use a custom value as their starting point.

• On each pricelist rule (the lines inside a pricelist), the "Based on" field now includes an extra choice called "Custom Value" next to the standard options.
• When a rule is set to "Custom Value", the price is no longer taken from the product's cost, sales price or other standard source. Instead, the system looks for a custom base price that another part of Odoo has placed in the current context; if none is provided, the base price is treated as zero.
• The pricelist price calculation has been adjusted so that this custom value is passed through when a caller supplies it, and the rule's computation uses it directly.
• If the "Custom Value" option is removed or no longer available, existing rules that used it fall back to the default base setting rather than breaking.

This module does not add menus, screens, buttons or reports of its own. It works behind the scenes on the product.pricelist and product.pricelist.item models and is meant to be used together with other modules that provide the custom value.