彼女の見ていた振袖は、彼女にぴったり似合いそうで、俺はせめて0がひとつ少なかったらと嘆くのだった。
I get the first part but I don't know what "0がひとつ少なかったら" means. At first I think is something like "0 becomes 1" but I really don't get it, even how the 少なかったら connects with it.
He's talking about the price of the 振袖. It's about the number of the 0's in its price.
He wished that there were one less zero in the price (tag). E.g. "1,000,000 yen" → "100,000 yen"
The たら indicates "(counterfactual) wish". E.g. 「車があったら(なあ)。」 "I wish I had a car!"