Item shops have done us the great service of asking, “How little do you value your time?” Putting a dollar value on what you are grinding for establishes that yes, you really are willing to do something you don’t especially enjoy for a reward worth a few cents per hour.
I mentioned League of Legends referrals lately. For each person who gets to level 5, you receive a 4-game IP boost (cost $2.23, although given the exchange between RP and IP, the value is about $1.00, so I assume people tend not to but those), and at 10 referrals, you get a free champion ($7.50). So a referral is worth about $1, maybe $2 if you can get 10 of them. You should put more effort into getting a free taco. Ah, but here is where it gets fun: people make multiple accounts and refer themselves to get the free champion. How long does it take to get to level 5? Not too long, I’m sure, and playing a few rounds of very low level LoL might even be fun, but people are going through that effort for the equivalent of $2.
See also: entering contests and drawings, where you can computer what fraction of a cent your expected value is. If you drive somewhere to pick up your lottery numbers, the gas you burned was probably worth more than the expected value of the ticket.
: Zubon
Item shops also helpfully raise the question of why you are paying to play a game where, given the option, you might pay to avoid playing (parts of?) the game.