The fare amount doesn't matter. As long as one segment in your itinerary was a fare type that required immediate united.com purchase (like many of the internet-only sale fares that are labeled something like "Fall Getaway Fare" and such)...then Book & Hold will not be available.

Many of these "Sale" fares often come up as more money than other readily available fares. So fare amount (high or low for the market) doesn't have any bearing on whether or not you get the B&H option offered.