Subject: Re: OT: SPY vs SPX options and IV
Ah, never mind. I was looking at the quotes from Yahoo.
Here's what I get from an actual broker.

April 17, 2025 call
SPX 5700.00 bid 236.40 ask 237.40 volume 200 open interest 2,668
mid: 236.90

XSP 570.00 bid 23.33 ask 24.20 volume 0 open interest 10
mid: 23.77

With SPX= 5470.10 and XSP=547.01, exactly 1/10'th

The XSP call is (perhaps) slightly more expensive. Makes sense, bigger players get a better price.