One-armed robber

The score adds up three things. Hours of play per dollar is worth up to 55 points, on a log scale so a 200-hour game isn't treated as twice as good as a 100-hour one. How far reviews sit above 60% positive is worth up to 35. The number of reviews is worth up to 10, so a handful of glowing reviews can't carry a game. Playtime is the typical playtime of people who reviewed it. You can't overpay for a free game, so its first 55 points come from how positive reviews are instead: none at 50%, all of them from 90%.