product_value = 0

pickings = self.env['stock.picking'].search([],limit=1)

for picking in pickings:
    for valuation in picking.move_lines.stock_valuation_layer_ids:
         product_value = product_value + valuation.value

If i understood you .. you are trying to get the inventory valuation layers for a specific product there is a method provided by odoo in every stock move that can help you get all the valuation layers keep in mind that you have to get the right stock pickings for your products if you want to get the accurate valuations.

Here is an example from Odoo that is using the same method in stock_account module 

class StockPicking(models.Model):
    _inherit = 'stock.picking'

    country_code = fields.Char(related="company_id.account_fiscal_country_id.code")

    def action_view_stock_valuation_layers(self):
        self.ensure_one()
        scraps = self.env['stock.scrap'].search([('picking_id', '=', self.id)])
        domain = [('id', 'in', (self.move_lines + scraps.move_id).stock_valuation_layer_ids.ids)]
        action = self.env["ir.actions.actions"]._for_xml_id("stock_account.stock_valuation_layer_action")
        context = literal_eval(action['context'])
        context.update(self.env.context)
        context['no_at_date'] = True
        return dict(action, domain=domain, context=context)