What I know about recording contracts back in that time would fit in a thimble, but let me speculate here. The problem with one hit wonders in the days of old were that the first contract a group got for a record was heavily loaded in favor of the producers and the record label. Later, if they could show some sustained popularity it translated into leverage in subsequent contracts, and much higher per record royalties. The irony in a lot of the 60’s nostalgia for groups that hit it once and then disappeared is that we can all remember the music, but the folks that created it died poor.
If I understand anything about current performance and record compensation it is that the per record compensation means very little - because of the incredible diversity of recordings that are out there and available - there has been an atomization of the market since the 60’s, 70’s, and 80’s. So live performances are a much bigger share of the total compensation of a performer than they had been 40 years ago.
Reading Lewisohn’s terrific Beatles biography - and he has a special interest in the business side - sez in 1963 that getting a recording contract was mainly to enhance ticket sales at live shows. Doubt it was much different in US as Capitol owned EMI.
Doing the math, the Beatles got about 1 cent per song (from each record sale). A bit before my time, but by late ‘60s (with 2% annual inflation) singles were about a buck (often discounted).
The money was in songwriting - a little over 4 cents per song (per record sale). So, say, Paul, was getting 1/4 cent for each single sold when the Beatles recorded a cover - and 2 cents when somebody covered a Lennon-McCartney creation.