Playing with spreadsheet. My CAL is 8154 and next premium cost is 15,500. If I reduce next premium cost to 8000, CAL increases to 8465

So it appears that buying premium expansions reduces CAL - hence an incentive to buy. Is my understanding correct?

Premium expansions do not reduce CAL, they merely have smaller penalty than the regular ones.

In your example you have 152 expansions total, with the next premium one costing 15,500. This means you have 36 premiums and 116 regular ones. If you drop price of the next premium expansion down to 8,000 this would correspond to 21 premiums and 131 regular ones for the same total of 152 (you can't even have 131 regular expansions, but it changes nothing in this analysis). So you replaced 15 premiums with regular expansions, and regular ones are penalized more, so your CAL went up.

To isolate impact of premiums alone, you have to drop total number of expansions correspondingly, so look at 137 total expansions with the next premium costing 8,000 (21 premiums / 116 regular ones). You will see that it will be lower than 36 premiums / 116 regular ones.