i would guess the fee would be broke down into a base rate charge then add on a percentage
£1, the charge is 23p, so say the base rate is 20p, then you have 3p/£1 = 3% (the %add on)
see if this is accurate with your second figure
£10. the charge is 52p, base rate 20p + (0.03*10 = 30p) = 50p (close to 50p)
so the percentage is prob done to a couple of decimal places and maybe rounded up to get the final figure which would work for these two figures
i dont know if this is how paypal do it? but it works for these two numbers
simples