auto price = prices.upper_bound(x); int canBuy = distance(prices.begin(), price);

The calculation of canBuy takes $$$O(n)$$$ time in the worst case instead of $$$O(\log n)$$$ as you suppose, where n = prices.size(), since the time complexity of distance(prices.begin(), price) is $$$O(n)$$$ in the worst case, because these iterators are not random access.

Time complexity of std::distance( InputIt first, InputIt last ) is constant only if InputIt is the random access iterator, else it is linear.