One is how is it priced compared to similar items.
I like this, particularly pairing it with the original question - "is TTRPGing an expensive hobby." This leads me to ask, "what other hobbies do I enjoy and how much do I tend to spend on them?"
I googled "common hobbies" and found the following:
Most popular hobbies & activities in the U.S. 2023 | Statista
Are TTRPGs more expensive than cooking/baking? (How do you price this? Solely on ingredients? On pans and mixers and pots? Do you factor in the oven/stove? The cost of renting/buying the house?)
Are TTRPGs more expensive than reading? (Probably yes, libraries can help a lot here to defray reading costs).
Are TTRPGs more expensive than pets? (Probably not. Pets plus food plus vet bills plus toys get expensive in a hurry).
Are TTRPGs more expensive than video gaming? (Probably not. A $70 game is usually what, 50 hours of enjoyment... you're probably likely to get more than a dozen sessions out of a $70 TTRPG book; also you can share your PHB with your buddies and play collaboratively - with Call of Duty each of you has to shell out for his own copy).
Are TTRPGs more expensive than outdoor activities? (Probably yes for hiking where the only costs are shoes, water, and maybe sticks. But probably not for anything else - skiing, boating, backpacking, camping, mountain biking - all of these will cost you a lot more).
Are TTRPGs more expensive than travelling? (Probably not unless your travel is VERY limited.)
Are TTPRGs more expensive than Socializing? (Probably yes, but aren't TTRPGs ALSO socializing themselves? Also, I suppose it depends on what your food/alcohol budget is when socializing vs TTRPGing)
Are TTRPGs more expensive than DIY Arts & Crafts? (Probably yes on an individual craft basis, but probably not if you're crafting weekly or biweekly).
Are TTRPGs more expensive than board games/card games? (Probably not if you're talking collectible card games or betting with your buddies in poker, probably about the same if you're talking about a wide variety of games, probably yes they are more expensive if you're talking getting a deck of Uno cards and playing the heck out of it.)
Are TTRPGs more expensive than Gardening and plants? (This one's a push depending on how much gardening you're doing and how expensive your plants are. If you get hardy perennials as a one-time expense and just keep them up, TTRPG is probably the more expensive bet. If you're re-planting your garden every season with flowers to match the holidays, it's gardening).
Are TTRPGs more expensive than cars? (Almost certainly not).
Are TTRPGs more expensive than sports/fitness? (Depends on the sport/fitness; your gym membership will probably wind up costing more long-term than a TTRPG book, and if you're into something like golf, clubs and green fees are a lot more expensive... but if you're just shelling out for a pair of shoes to jog in once or twice a year, TTRPGs cost more).
So... I think the answer is "it depends on exactly what hobbies you substitute for TTPRGs... but TTRPGs are probably not really all that expensive a hobby." Of course there are some versions of hobbies that will obviously be cheaper than TTRPGs (Uno cards, jogging, reading), but most common hobbies look like they would be a push with TTRPGs (board games, baking, arts & crafts) if not wildly more expensive (especially travel and cars, but even playing poker with your buddies might be more financially stressful).