The formula is quoting the returned digits: it returns "1" instead of 1. I would change the formula from
If(
0 <= :Semester donations estimate <= 10699, "1",
10700 <= :Semester donations estimate <= 35199, "2",
35200 <= :Semester donations estimate <= 70599, "3",
70600 <= :Semester donations estimate <= 113899, "4",
113900 <= :Semester donations estimate <= 162999, "5"
)
to
If(
0 <= :Semester donations estimate <= 10699, 1,
10700 <= :Semester donations estimate <= 35199, 2,
35200 <= :Semester donations estimate <= 70599, 3,
70600 <= :Semester donations estimate <= 113899, 4,
113900 <= :Semester donations estimate <= 162999, 5
)
and have a numeric column.