Honestly, money is worth nothing to me. Which might be surprising seeing as I'm a student ATM without a big income, who is struggling to save for many things he'd like to do, but honestly money is unimportant. I know this probably won't directly address "How much is $40 worth?", but hey, that's a meaningless question to me as $40 and $100 is worth nothing to me. Its what I can buy with that money that is worth something, and what each of those is worth differs depending on my situation and what I want to do at that time.
Money itself though has no intrinsic value. I'd be happy to pay $100 for a light lunch, and I have at times, if I feel there is nothing that I want to do that that cost will lock me out of. I'm happy to buy a game I want for $300 if there's nothing else I would want to spend that money on at the time. What matters to me is in the moment, and mid term future. If, looking over the next couple of months, there isn't anything that I won't be able to afford that I want to do - I'll spend as much money as I feel like at any given time. If there is, I'll restrict that money useage to a level that I'll be able to afford doing that thing.
This may seem short sighted, and not giving adequate consideration to the future, but its a necessity of life for me. I'm presently seeing a psych because I highly likely suffer from anxiety and depression. There's obviously a lot of complications there, but one big effect for me is that I have little to no motivation or desire in my life in general, and I'm fairly prone to suicidal thoughts and attempts. For me, planning for 10 years in the future is like planning for the afterlife - I don't even know if its real, if I'll get to see it. I'm also rather unstable a lot of the time, so keeping myself happy short term drastically increases the odds that I'll have a long term, so focusing on saving for the future is kind of like selling my car to buy some fuel.
So how much is money worth to me? Nothing. $40 is worth a similar amount to $100. I'm not going to be stupid and trade $100 for $40, and if there is a way to get the same damn thing for a cheaper price, thus giving myself more ability to do other things, I'll do that, but I don't consider things in terms of prices. Merely in terms of how much I want to do something or not, and how much doing so will lock me out of doing other things I really want to do. What matters to me more than getting good value for money, is having a constant stream of either distractions, or enjoyable experiences, that keep me afloat until I get better - if I ever do. Because of that, I'll pay whatever I need to at the time, so long as it won't destroy my ability to do so in the short-mid term future.