From LBPWiki

LBP has a fairly complex scoring system that awards each player points based on collection of score bubbles, prize bubbles, creature brains and, in LBP2, the score giver and killing another player in competitive mode.

There are several ways to accrue score.

• Score bubbles are worth 10 points each.
• Prize bubbles are worth 50 points each.
• Creature brains are worth 50 points each.
• Score givers give (or take) the score set in their tweak menu. Although the score giver can take points away (-) the players overall score cannot be reduced below zero (0).
• Killing another player in competitive mode earns you 10 points.

Combos

Collecting score bubbles, prize bubbles and creature brains in quick succession fills a combo meter, that appears as a white circle over your score in the top left of the screen. Every 5 bubbles/brains collected increases the score multiplier by 1. Combos can be added to or maintained by more than one player, which then multiplies all points acquired by each player individually. Combos count all score bubbles which have been collected after completion on the last combo. For example, if a player was to collect one score bubble, and then later create a 2x combo with 5 score bubbles, they would be awarded 120 points (6 bubbles in a 2x combo).

Note: In LBP2, combos do not affect scores acquired from score givers, or in competitive mode. However, after the 1.06 patch, they can be tweaked to allow combos.

Death

In normal gameplay modes, dying causes 5% of your score to be deducted. In LBP2's competitive mode, dying as a result of your own action causes your score to be reduced by 10 points. If you use the retry button in LBP1 or LBP PSP, you do not lose any points.