This will be tough to prove either way. In 2014, I was told by an industry insider that GPI and CPC chips were about the same. More recently (last month) I spoke to someone who knows the cost of GPI chips to casinos. He would not give me an exact cost per chip, but I told him what Key West chips cost.
I asked what GPI was charging these days. His response: "They're really, really expensive."
The "buck a chip" guess was a ballpark maybe 12 or more years ago. I think the guess of $1.25 per chip is low for the same reason. The base cost of materials is higher than you would think.
$1.50 @ may be possible if you are buying many, many chips.
I wish I knew what GPI charged per chip these days.
The casino where I worked had a minor change in the name. They changed the logo on the playing cards, but kept the old, worn, sticky GPI chips. The dealers would always ask when we would be getting new chips with the updated logo.
The casino bosses said new chips were "too expensive."