Just like eg6sir.....this is just my opinion.
Your rims with tires are around $650-725 (brand spankin' new). But, in all honesty, w/semi new tires (>75% tread left) and some curb-age, they are worth $375-425 to a buyer who wants them.
So $50 is pretty close.....if and only if the tires on the Si's will last you 15K+ miles and they are truly 9/10 (or better) rim condition. Come back at $100 and see where he goes.
I see guys "try" to get $400 for 99/00 Si's with mediocre tires (they never got 4 good ones lol) But I agree that they are worth $300-350....maybe $375 if rims are this minty (I am skeptical)