The way that D&D is set up though, it seems that the DM is assumed to wear a lot more of the financial burden. Even at a very base level, the DM is expected to buy 3 books (PHB, DMG and MM), whereas the players only have to buy the PHB.
So what do you think? Does the way D&D is set place too much disparity between the DM and the players in terms of cost? How is the cost spread in your group?
I've quoted this part of your text as the majority of replies focuses on a personal level.
Judging from my personal experience, the disparity in costs between DM and players just reflects the disparity in levels of commitment.
Many (of my) players see RPGs as a nice pastime and enjoy the social aspects and the story of an adventure more than the technical parts of the thing. As such, they don't have a lot of interest in hundreds of spell descriptions, and avoid even the PHB. Maybe they once bought a PHB or whatever is needed for any other game system and noted that they didn't actually use it. So why should they buy even more of the stuff?
TSR once tried to answer the question of what they could sell to players
The GM demonstrates a greater commitment by virtue of taking on the task. As long as he's no miser or coin-shy I easily see him buying into the game. Also his interest covers more ground. While the players would need basic rules and information on their race and class - to stay in D&D lingo - the DM has to have (access to) all that, but background, monster, and adventure information as well.
I can only remember three players who bought more than the absolutely necessary stuff:
One, a master of system mastery, bought the books to be able reach his desired level of achievement.
The second, a rather well-to-do guy, bought a lot of books to have them and maybe leaf through them in an idle minute.
The third one, finally, is currently running 4e for us. She had offered to run a 3.5 game several years ago and went on a shopping spree. Now she has repeated the procedure for 4e. I can tell you she isn't keen on 5e!
