Hi,

Not sure with your logic, you can correct as per your use case, define the field as a compute field, then

    cost = fields.Float( string="Cost", compute='_get_cost')

Then define the compute function,

  def _get_cost(self):

        cost_rec = self.env[ 'course'].search([], limit=1)

        if cost_rec:

                self.cost = cost_rec.cost

the above code will search for the records in the model named course and return a single record. As per the logic you can add search condition in it.

Thanks