tsql query analyzer : how do you reduce "cost"?

I'm running sql analyzer on the following query

SELECT bl.Invoice_Number, bl.Date_Invoice, ti.TranNo, bt.Description,
    CONVERT(decimal(15,2), bl.Invoice_Amount) AS Invoice_Amount, co.Company_ID, co.Account_Nbr,
    isnull(bl.Reference,' ') as Reference, bl.Billing_Log_RecID AS BillingKey
    FROM [CONN.domain.NET].cwwebapp.dbo.Billing_Log bl
    LEFT JOIN [App].dob.tarInvoice ti
        ON bl.Invoice_Number = dbo._fnStripLeadZeros(ti.TranNo)
    INNER JOIN [CONN.domain.NET].cwwebapp.dbo.Billing_Type bt
        ON bl.Billing_Type_ID = bt.Billing_Type_ID
    LEFT JOIN [CONN.domain.NET].cwwebapp.dbo.Company co
        ON  bl.Company_RecID = co.Company_RecID
    WHERE bl.Date_Invoice >= '2009-05-05'
        AND ti.TranNo IS NULL
        AND bl.Invoice_Amount <> 0
        AND bl.Billing_Type_ID <> 'D'
        AND bl.Billing_Type_ID <> 'P'
--      AND bl.Billing_Type_ID <> 'M'
Order By bl.Invoice_Number

The query runs on the [App] server and connects to [Conn] sql server to do a join and the diagram is telling me

remote query cost : 62%
customered index scan [App].[dbo].tarInvoice.[PK__...  Cost : 34% 

This query is taking 2 mins to run. Any ideas on how would I go about figuring out how to make this run more efficiently? I"m guessing it has to do with connecting to another sql server on the same network.

thanks in advance.

you are doing a complete scan of the tarInvoice clustered index (touch every index entry), see if you can remove the function call dbo._fnStripLeadZeros(ti.TranNo) so it will use the index.

possibly add leading zeros onto bl.Invoice_Number and join to unaltered ti.TranNo

EDIT

add computed column without leading zeros and add an index:

ALTER TABLE dbo.tarInvoice ADD TranNoZeroFree AS Convert(int,TranNo) PERSISTED 
GO
CREATE NONCLUSTERED INDEX IX_tarInvoice_TranNoZeroFree ON dbo.tarInvoice (TranNoZeroFree) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO