"It will be more of a sucks for him ..."
I've seen the disparaging sense of sucks as a verb ("sucks to be you", "that sucks!"), but this particular usage from Price Caspian seems a little odd:
Lucy heard Edmund say, "No, let me do it. It will be more of a sucks for him if I win, and less of a let-down for us all if I fail."
My interpretation was that It'll be a sucks for him is intended to convey "It will be a victory for us", with strong implications of humiliation and embarrassment for Trumpkin.
However, I can't find a dictionary definition which quite matches this use of the word.
eat crow, Informal. to be forced to admit to having made a mistake, as by retracting an emphatic statement; suffer humiliation: His prediction was completely wrong, and he had to eat crow.
Solution 1:
According to the OED, sucks was an interjection of contempt for children from the early 20th century:
11. pl. as int. Used as an expression of contempt, chiefly by children. Also in phr. sucks to you and varr. slang. 1913 C. MACKENZIE Sinister Street I. I. vii. 98 This kid's in our army, so sucks! 1922 F. HAMILTON P.J.: Secret Service Boy iv. 178 ‘S’, he announced, ‘u,c,k,s,t,o,y,o,u.’ 1935 N. MITCHISON We have been Warned I. 28 Brian is a baby. Oh sucks, oh sucks on Brian. 1945 E. WAUGH Brideshead Revisited II. v. 287 It's great sucks to Bridey. 1952 ‘C. BRAND’ London Particular xv. 191 A most regretable air of sucks to you. 1968 Melody Maker 30 Nov. 24/5 This is a rotten recordyah boo and sucks. 1974 Times 4 Mar. 9/5 Sucks boo, then, with acting like this, to that new National Theatre down the road. 1978 ‘J. LYMINGTON’ Waking of Stone ii. 45 ‘Sucks to you!’ she said..tossing her head so her pigtails swung. 1983 Listener 19 May 11/1 The council treated the urbane Mr Cook to the politician's equivalent of ‘Yah, boo, sucks’.
These are all notably British quotations.
Solution 2:
This usage appears memorably in William Golding's novel Lord of the Flies (1954):
"I can't swim. I wasn't allowed. My asthma—"
"Sucks to your ass-mar!"
The speakers in this dialogue are British schoolboys. As in Hugo's answer, the intention is clearly to express contempt. Sucks is used repeatedly throughout the novel and becomes something of a catch phrase.
I think this is distinct from the modern usage of suck as a verb. The noun seems to be much more mild, as it was chiefly used by children. In contrast, the modern suck started out quite offensive (as a reference to fellatio) but has softened over time and been picked up by children and teens.
Solution 3:
So far as I can see, OP's citation from C.S.Lewis's 1951 Prince Caspian is a complete one-off. Or at least, that 'noun' usage probably hasn't existed for generations - it's invariably a verb today.
Googling today's Internet, I can't find a single instance of "a sucks for me" used in this noun sense, as in "The whole day was a sucks for me" (but there are plenty for "The whole day sucked for me").