Eichel is better than Marner and plays C so 100% has more value, if you think otherwise then you are simply wrong.

That being said if Eichel demands a trade the leverage is limited and most teams will offer at most:
A prospect + 2 B prospect + 1st+ 2nd. I'd be shocked to see any team offer more than that for a player who has caused some headaches for his organization, is injured and hasn't made the playoffs. (Still a top 10 NHL player but has shortcomings)

Starting with the finances, if the deal takes place after July 1st, Buffalo takes on 900k against the cap for 4 years and frees up a 5th year. They also save $15.2M in real cash even with $2M per season retained. So for a club with an internal cap that can't hire staff at the moments it is a lot of money.

Going to the hockey: Toronto gets a top 10 player at a cheap cap hit for 5 years.

Buffalo gets a play that finished top 5 in NHL scoring. You can't completely go for future given the money invested in Skinner and others today so this gives hope that the team can become a playoff contender in the next 2-3 years with Marner/Skinner under contract. You also get 2 prospects rated in the top 100 in the NHL by NHL.com. Both played last year in Europe and are expected to become NHLer. Hallander also played last year and could be a good 3C in the future, a bit riskier than Niemela and Hirvonen. The 2nd adds to a pool of picks to try and find some gems in a lottery of a draft.

Overall Toronto gets better and saves 2M vs the cap.
Buffalo saves $15.2M in cash, swaps a Top 10 NHL player for a Top 25 NHL player and picks up two B+ prospects and one B- prospect along with a second.