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.
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.