Update With Different CAST And CONVERT

I have a table with all varchar data for all different fields for money and int etc. I want to do a calcuation in one field with the rule as below:

UPDATE [dbo].[tblPayments]
SET [PreInjuryWage]=
CASE
WHEN [WeekNo]<14
THEN ([MaxWorkCover]+ [WeeklyEarnings] )/0.95
ELSE ([MaxWorkCover] + 0.80 * [WeeklyEarnings] ) /0.80
END

I tried this command after many other ways of tries

UPDATE [dbo].[tblPayments1]
SET [PreInjuryWage]=
CASE
WHEN [WeekNo]<14
THEN CAST ( (cast([MaxWorkCover] as money )+ cast([WeeklyEarnings] as money ))/0.95 AS Varchar(10) )
ELSE CAST ( (convert(money, [MaxWorkCover] + (0.80 * convert(money, [WeeklyEarnings] )) )/0.80 ) as varchar(10))
END

The structure of the table ( its a legacy table from 2000 i beleive)

CREATE TABLE [dbo].[tblPayments](
[PaymentID] [int] IDENTITY(1,1) NOT NULL,
[ClaimID] [int] NOT NULL,
[WeekNo] [smallint] NULL,

[code]....

Arithmetic overflow error converting numeric to data type varchar.