2002 Volvo XC70 slight bucking during downshifting and putt

The first thing you may want to try and do is have the Transmisson Control Module software updated. If there is one available it usually helps with some shifting issues. Most likely you will need to go to Volvo for the update. If you do need a transmission my guess will be somewhere around $3000 to $5000 depending on the shops hourly rate and where and how much you get the part.