When I play a new game, there are only five things I give a shit about. I use them to make a formula. This formula tells me, in dollars, the most I should pay for a game.
Is the game good, very good or GOTY tier excellent? This is A
How long is the base game in hours? This is B
What is the ratio of optional / post game content / 100% content vs base game? (i.e. 0.25 if extra content is 1/4 of base) This is C
How good is the post game content vs the base game? (total shit or not applicable (in the case of roguelites like BoI), quite weak, just as good, better, much better, assigned 0, 0.25, 0.5, 0.75 and 1 respectively) This is D
How replayable is the game? (not at all, fairly replayable, very replayable, extremely replayable/core point of the game assigned 1, 2, 5, 25 respectively) This is E
I assign a score of 1, 2 or 3 to whether the game is good, very good or GOTY excellent. I use user reviews from metacritic exclusively here and require a minimum of 50 user reviews. Critic reviews are not worth the paper they're printed on. User reviews of 8.0 - 8.5 are good, 8.6 - 9.0 are very good and 9.1+ are GOTY excellent.
Look at this link: metacritic.com
Using this system, despite critic reviews of GTA IV being very high, the user reviews average below 7.9 so I know that this game isn't good. I've unfortunately played it and can attest to the disparity between user reviews/actual experience and critic reception.
The formula I made is : ( A ( B ( 1 + (C x D) ) ) E
Let's take a look at a game, say Super Mario Galaxy.
A is 3. B is 15 (taken from howlongtobeat.com
So we have (3 ( 15 ( 1 + (2 x 0.25) ) ) 2 = 3 x 15 x 1.5 x 2 = $135. i.e the game is well worth $60.