If you leave this right till the end of the game you can just click through them all in one run as you'll have more points than you actually need.
On the main menu under the garage option is where you'll the unlockable bonus content. Choose the option on the left for game.
The are four sub-headings; Photos, Exclusive, Audio and Concept Art.
Photos has 55 items to unlock at 250 points a piece which totals to 13750.
Exclusives has 22 items to unlock at 350 points a piece which totals to 7700.
Audio has 28 items to unlock at 200 points a piece which totals to 5600.
Concept Art has 420 items to unlock at 150 points a piece which totals to 63000.
When you have purchased all of these items at a total price of 90050 you need to go back to the main menu for the achievement to unlock.
If you have also purchased all of the 'Film' unlockables you will also unlock the achievement for Behind the Scenes Bonus DVD