This Wikipedia article, as well as IMDB, say that Damnation's budget was $17,000,000:
https://en.wikipedia.org/wiki/Damnation_Alley_(film)#Production
https://www.imdb.com/title/tt0075909/
“This Wikipedia article, as well as IMDB, say that Damnation’s budget was $17,000,000:”
Yes, and that’s the same article I used - but based on the book called Twentieth Century-Fox: A Corporate and Financial History (The Scarecrow Filmmakers Series). The figure you cited is cited in several places on the internet but not in the book. The only conclusion I can come to is that the book actually used the financial records of 20th Century Fox and, therefore, is more likely to be correct.
The comparison of Damnation Alley with Star Wars based on budget is not uncommon: https://www.popmatters.com/144633-damnation-alley-2495988495.html
As Solomon wrote:
“The only real disappointment of 1977 was DAMNATION ALLEY, a more routine post—nuclear war, science—fiction adventure which was a total loss on costs of $7.5 million. DAMNATION ALLEY, released just before STAR WARS, was the final fizzle to the standard sci—fi movie.”
In fairness, I should point out that Solomon says Damnation Alley was released before Star Wars, but the general release date was in October and one source said it was held back precisely to give more of an uplift to Star Wars in May of 1977.
In any case, Damnation Alley did not have budget twice that of Star Wars.