Found Solution This is what I want to do

 from openerp.osv import osv,  fields
import openerp.addons.decimal_precision as dp
from openerp.tools.float_utils import float_compare


class company(osv.osv):
    _inherit = 'account.tax'
    _columns = {
        'Tax_Included_In_Costprice': fields.boolean('Tax included in cost price',
            help="Do cost price include tax?"),
    }

class purchase_order_line(osv.osv):
    _inherit = 'purchase.order.line'
    def _compute_total(self, cr, uid, ids, prop, arg, context=None):
        res = {}
        cur_obj=self.pool.get('res.currency')
        tax_obj = self.pool.get('account.tax')
        for line in self.browse(cr, uid, ids, context=context):
          unit_price = line.price_unit
          if line.item_seller_price != 0:
            unit_price = line.item_seller_price
          taxes = tax_obj.compute_all(cr, uid, line.taxes_id, unit_price, line.product_qty, line.product_id, line.order_id.partner_id)
          cur = line.order_id.pricelist_id.currency_id
          res[line.id] =  cur_obj.round(cr, uid, cur, taxes['total_included']-taxes['total'])
        return res