Arthur/Solomon are 300 cause they're more the basic-legendary souls I guess. As you start the game with Lancelot, they make it easier for you to be able to get Arthur and Solomon, since she's the standard souls needed for both of them.
Others are 500 I guess cause you're to get them a little later in the game? Though Andromeda was my first... they're more for strategy like long battles, tank soul like Arc, debuffing enemy by Mordred, etc.. rather than just giving your partners atk/ability damage buffs which are useful at the start of the game, still are now cause Arthur's ATK buff is high...
(though I completely went against that, Arthur and Solomon were both the last legendary souls I got lol...)
Soon we'll have another that's 600 SP and it's Hercules.. upgraded of Spartacus/Achilles