Still, I can understand the sentiment. There's a dirt-common poly saying: "Love may be infinite, but time is not." You can substitute money for time and it's no less true.
I am a man naturally given to the extravagant, when the extravagant fits within my budget. I love to give, and while I have very much internalized 'it's the thought that counts'. my thoughts for loves and friends run to spoiling them.
My budget does not run. My budget occasionally limps out to the kitchen, looks around, and slinks back to the couch in despair. And yes, sometimes that bothers me. I've never been anything close to 'wealthy', materially; truth be told, I wouldn't necessarily want to be. Comfortable, is what I aspire to. And I've had to redefine comfort a few times. Largely successfully, but with occasional wistful moments that force me to reframe my thoughts.
In a poly context, it becomes just one more insecurity to overcome: he's richer than I am, he can give her
Negative thought, perish the negative thought. Reframe, reframe, reframe!
Maybe he can take her halfway around the world and I can't even get her out of the province. But I have one thing, one obviously desirable thing, that he doesn't: I'm me. Our experiences, all of them, are fundamentally different because of that fact. Note again: there is no judgment, positive OR negative, implied in the word 'different'. His experiences with her are no more or less valid than mine are.
And rich or poor, you can't put a price on unconditional love.
Actually, there are some aspects of polyamory that make it very well suited to LOWER class people. A sufficiently close polycule shares finances, or if that step hasn't been taken, at least may have access to emergency funds. Calamity is spread out, both emotionally and financially, such that any one person isn't expected to bear any one other person's burden. That is immensely liberating, and it makes for a nice Scotiabank commercial: you're richer than you think.
Never mind emergencies: sharing expenses amongst a larger number of people makes available experiences (and things) that may otherwise be out of reach. It takes compersion, a great deal of it, to take some of your money and give it to your partner to spend on their partner. Knowing that reciprocity exists -- your partner, or hell, your metamour, does the same thing -- makes it much easier.
Yes, you run the risk of being financially taken advantage of. This is no different than in monogamy; "gold diggers" are, after all, a thing. That comes down to selecting partners who are not assholes.
What I have found is that love flourishes not in the grand gestures, but in the mundane, quotidian details of life. Whether you go to bed on an air mattress in a bare room or on a bed adorned with 17,000-thread-count unicorn hair sheets, cuddling and drifting off to sleep together is pretty much the same. Great joys can be had as easily on a Sunday drive as they can be on a weekend jaunt to Paris (then again, what with the price of gas, perhaps a Sunday drive isn't the best example to use here).
Rich or poor, we live and laugh and love in our own unique ways. And if we are poly, we cherish each other, and all our partners, and ideally all our metamours too, for that unique love.