The free MCP days would be half price with Mammoth Premier. You need to ski 3 other MCP resorts for the MCP to be the better deal.
Mammoth has adopted the Vail philosophy of jacking up day ticket prices to induce people to buy season passes or other advance promotions. So half day at Mammoth under MCP is in the $65 range for weekends now. So if you're only skiing Mammoth it takes 6 free days on a Mammoth Pass vs. being half price with MCP to make the Mammoth Pass the better deal. So that means 9 days total assuming you're choosing Mammoth for the 3rd free day on MCP. For every extra MCP resort you visit add 2 to the number of required days to choose Mammoth Pass.
5-8 days at Mammoth MCP is better deal even if not skiing other resorts.
9+ at Mammoth buy Mammoth Pass if not skiing other MCP resorts
11+ at Mammoth buy Mammoth Premier if skiing 1 other MCP resort
13+ at Mammoth buy Mammoth Premier if skiing 2 other MCP resorts
If skiing 3+ other MCP resorts always buy MCP. Buy Mammoth Pass if skiing 9+ days at Mammoth
I've had a Mammoth Pass for a decade and it's been Premier the past 3 seasons. My ski days at Mammoth have been volatile so some years it hasn't paid. Last 10 years Mammoth days: 16,3,4,15,12, 25,9,18,3,4. I have 8 so far this season and may get another 8 by the end of the season. I ski Alta/Snowbird every season and usually one other MCP resort (sometimes more). While the last 2 years would have been better with MCP, this is not predictable in advance for me. At any rate we hedge our bets by my getting Mammoth Premier and Liz getting MCP.
EDIT : I had the walk-up rate wrong in the following text. It's $135, so Tony is correct on what the half-price tickets would be.
I'm not sure if this changes that calculation, but isn't the MCP deal 50% off the walk-up price, not the online advance price? (I believe it was at Aspen, as I paid $75 for a half-price weekend day.)
In March (and presumably earlier) the 1-day weekend non-advance price at Mammoth was, I think, $170. So the MCP half price deal would have been $85. Right now, it appears to be ~$160, so half price late season would still be $80.