If 1L cost $1.25 then how much would 1cup cost? How would i do this?

What kind of cup?

A coffee cup, a cup o' tea?

There is such a thing as the "metric cup" where 1 cup = 250 ml
In that case 1 cup would cost 31 cents

1 U.S. customary cup = appr 236.588 ml
In that case 1 cup would cost
$1.25 ÷ (1000/236.588) = $.295 or 30 cents

1 U.S. "legal" cup = 240 ml
so 1 cup would cost 1.25 ÷ (1000/240)
= 30 cents

1 Imperial cup = 284 ml
In that case 1 cup would cost 1.25 ÷ (1000/284)
= 36 cents

1 Canadian cup = 227.3045 ml (no longer used except in old cookbooks, we have gone metric since the 1970's)
In that case 1 cup would cost 1.25÷(1000/227.3045)
= 28 cents

There is also something called a Japanese cup ,
the point is, GO METRIC and avoid such silly problems.