It is a bit hard to come up with a valuation that makes sense on this cover.
Robert Rockwell was a pilot in the famous Lafayette Escadrille N.124 and that is a plus, however, on the minus side he did not shoot down any enemy planes. Collectors generally are willing to pay premiums for covers from pilots with 'kills' to their flying record, and even higher premiums for pilots who were ranked as 'aces' which meant they had at least 5 kills to their credit. For example, if this cover were from Kiffin Rockwell who unlike Robert was one of the original members of the Lafayette Escadrille (and who had the same last name) it would have had a good premium since he was the pilot who shot down the first enemy aircraft after this Escadrille went into action. And also, sadly, Kiffin would get an even bigger bump since he was shot down and killed in the line of duty which morbidly makes him a sought after pilot to many collectors. Or better yet, someone like Raoul Lufbery who was very famous personally as the squadron's 'ace of aces' with 17 of the squadron's 40 total victories would have a very large bump up in value.
Unfortunately, Robert Rockwell was not only a pilot without a credit of shooting down an enemy aircraft, he was more known for enemy pilots taking target practice on his aircraft. In two instances his aircraft was so shot full of bullet holes that it was a miracle that he got his machine back to his base without crashing.
Another minus is that this cover features an American Expeditionary Forces (AEF) censorship and cachet. The Lafayette Escadrille was a unit in the French air force and most collectors would be expecting to see a French censorship instead. It was censored by an officer named Bryant. There were no officers by the name of 1st Lt. Bryant in the Lafayette Escadrille so that is a bit of a mystery. And officers did not censor other officer's mail. They censored their own mail so that is another puzzle.
Finally, there is a minus in the cancellation being incomplete in a critical way - the year is absent apparently from it being struck as a slight angle so the year slug did not leave an impression. It does have the month and day - December 24th - and someone, perhaps the recipient. hand wrote a date received of January?? 29?? 1918 which would suggest that the missing date would have been 1917 but most collectors want to see the date in the cancel not in a handwritten note. If it was mailed on December 24, 1917 that would have been at the very end of the Escadrille as it was transferred to the U.S. Air Service as the 103rd Aero Squadron at the beginning of 1918.
So, overall it is a desirable cover for its connection to a member of the Lafayette Escadrille, it is not something that would have large premiums attached. Covers from US pilots without any victories and from other units during the war can be found in the $3 to $10 range depending on the unit. My very rough guess is this might be more in the $15 to $25 range but it could go higher or lower. An
ebay auction started at a low number to encourage bidders to bid against each other might be the best way to find out what it would be worth.