Easy.
1. Sell it as a massive savings since the savings > costs. People never remember the historical details. Doesn't matter that $750 million has been spent if you keep saying you're saving $X billion.
How does "$750M in sunk costs" support that when Joe Short Attention span's head has already exploded at "$750M in sunk costs. . ." way before you get to the second part.
That's not how citizen psychology works re: government spending.
Ever. They get pissed at the wasted money, not the money that wasn't wasted on top of what was. If you're not convinced, try reading the Herald for 3 consecutive weeks and come back with one example--any example--of "Well...it could've been worse" succeeding at placating the outrage over misplaced gov't finances or priorities.
(Hint: you won't find a solid real-world example of one.)
2. Run a PR campaign to smear support for the project goals. Tout how useless it is for most the state. Tout the fiscal responsibility of cancellation (doesn't matter if it's actually true).
You can smear the cityfolk all you want, but there's nothing in the fiscal responsibility dept. to upsell it on because of #1. You're right; it doesn't matter if it's actually true, because everyone tuned out at the words "$750M in sunk costs".
Done. First maneuver sets the tone of the dialog as savings. Second maneuver sweeps out the oppositions legs.
No. It horribly backfires because you've lost the mob at the lede without ever having the chance to set a tone--any tone--in service of a purpose. That's how issues of outrage over gov't waste ding the public's medulla oblongata. Always. There is no second pivot move unless it involves telling them they're getting something real salvaged out of the unfavorable price, and stepping them down from the ledge. "You get nothing for a steep price, but at least it's not a steeper price" isn't a pivot, it's pouring lighter fluid on the first-reaction outrage over wasting
any money.
If you think there's a way to easily sell this on the uniformed exurbanite, it needs to jibe with Voter Psychology 101. And trying to appeal to voters who hate gov't waste by upselling a gov't waste doesn't jibe with that psychology. None of them care that the waste wasn't worse when they've already turned into spittle sprinklers over the waste being bad in the first place.