I don't think there are hard and fast rules on SP salaries.
But my own experience is a strong
Don't Spend A Lot on SP.
I'm playing a league now, for example, and we're 2/3 the way through the season in an $80m format, and one owner spent around $12 million each on his 4 SP*. We're talking primo, top-dollar Greg Maddux, Pete Alexander, Walter Johnson, and Sandy Koufax. He's also based in Forbes Field '09. I admire his approach, but he's 2 games under .500.
Now, he could come back, and I kinda hope he does, because I like good pitching. But right now, it doesn't look good.
https://365.strat-o-matic.com/team/1716246Fast, short, and sweet advice? Find old Negro League SP who don't give up HR. There are a bunch of 'em. NeL SP* are significantly underpriced.