Here is Andy Benoit from Sports Illustrated on his assessment of the trade:
Though the Rams led the league in scoring last year and finished 11-5, a persistent problem they had was defenses not feeling compelled to double-team Sammy Watkins, who often aligned on the weak side. Instead of helping on Watkins, the safety to that side would help inside against L.A.’s downfield crossing routes (usually run by Cooper Kupp). That help won’t be available now because Cooks has north/sound speed that demands a safety stay over the top. He also has the quickness to hurt you on east/west routes, which will allow him to be productive from any spot in the formation (Watkins didn’t necessarily offer this dimension). Our Albert Breer says Rams head coach Sean McVay has long been fond of Cooks and explored trading for him last year, before the Saints traded Cooks to New England.
The downside for L.A. is that Cooks’ rookie contract expires after this season. Retaining him will cost either $16 million-plus on a franchise tag in 2019, or much more than that on a multiyear deal. There’s a good chance they’ll find the cap space, but still: Over the next four years, the Rams will pay Cooks significantly more than they would have paid to whomever they drafted at No. 23.
Cooks will bring in another dimension that the wide receiver corps did not possess prior to Watkins being on the roster. The Rams have a big-play threat that can score everywhere on the field and will require the defense to respect him as a threat and allow the other receivers to roam free.
Because of Watkins being on the field last season, Kupp and Robert Woods were able to attack defenses. Even though Watkins finished with less than 600 receiving yards, Kupp finished his rookie season with 869 yards and five touchdowns. Woods was on board for 781 yards and five touchdowns. Cooks will help do the same.
Cooks is also the most decorated receiver on the roster:
#Saints fans don't agree that Brandin Cooks is humble, but can't dispute the production or reliability either. The four-year pro has not missed a single game since his rookie year and has posted 1k-yard, 7-TD campaigns each of the past three seasons. He will really help the #Rams— Jordan Schultz (@Schultz_Report) April 4, 2018
Maybe the asking price for Cooks was high, but if the Rams re-sign him then the trade will have been worth it and for the next 3-4 years, the Rams will possess a really good receiving corps.