Hi Tansel
A common cause for 'wrong' selling prices being exported is due to duplicate entries in the price matrix. These are not visible to the user. Basically sometimes when importing a price update, rather than updating, MYOB creates a new entry. When you view selling prices on screen, MYOB retrieves one of the prices. When you export, it retrieves a different one. Who knows which price it will use when you generate an invoice.
Suggest you get MYOB to run duplicate price matrix script on the online file and see if this resolves.
Regards
Gavin