If Statement with multiple conditions?

You may want to consider using CASE.

Case([state abbreviation],"NJ",30,"NY",30"MI",20)

To use the IF statement, I would do it like this:

If([state abbreviation]="NJ",30,
[state abbreviation]="NY",30,
[state abbreviation]="MI",30
)

Using  Case you can list the values what dwhawe  said is correct .Option has a value. You can't use nested cases 

Case(
  [state abbreviation], "HI", 30, "ME",30,"MI",30,"MO",30,"MS",30,"NE",30,"NJ",30,"NM",30,"OH",30,"OK",30,"SC",30,"UT",30,"WV", 30,
"AZ",45,"CO",45,"FL",45,"GA",45,"IA",45,"ID",45,"IN",45,"MA",45,"MD",45, "MT",45,"NC",45,"ND",45,"OR",45,"VA",45,"WY", 45,
"AL",60,"AK",60,"AR",60,"CA",60,"CT",60,"DE",60,"IL",60,"KS",60,"LA",60,"MN",60,"NH",60,"NV",60,"NY",60,"PA",60,"RI",60,"SD",60,"TN",60,"TX",60,"WI", 60,
"KY", 75 )

Default value is null , if you need default value after 75 just type a value without quotes 


Another Solution

Instead of that i think you can use script. 

Create a text field with From list options: State Abbreviations 

Create another text field : Value for Abbreviation

Using Image on load technique call the JS page on to the form

Write the Code like

Using on Change  functionality , check the value of the State Abbreviations field , get the value from the State Abbreviations Field, using  switch case define the three cases and  values for each  cases  Take that value using id , and  store the value to the Value for Abbreviation Field .

using J query Properties make  the  Value for Abbreviation Field as read only.

Code :

You need to change in code according to field id's
_fid_38 -> id of State Abbreviations Field
_fid_37 -> id of Value for Abbreviation Field

$("#_fid_38").change(function(){
var caseName = $("#_fid_38").val();
switch(caseName){
    case "HI":
    case "ME": 
    case "MI": 
    case "MO": 
    case "MS": 
    case "NE": 
    case "NJ": 
    case "NM": 
    case "OH":
    case "OK": 
    case "SC":
    case "UT": 
    case "WV": 
    $("#_fid_37").val(30).prop("readonly", true);
    break;

    case "AZ": 
    case "CO": 
    case "FL":
    case "GA":
    case "IA":
    case "ID":
    case "IN":
    case "MA":
    case "MD": 
    case "MT":
    case "NC":
    case "ND":
    case "OR":
    case "VA":
    case "WY": 
    $("#_fid_37").val(45).prop("readonly", true);
    break;

    case "AL": 
    case "AK": 
    case "AR":
    case "CA":
    case "CT":
    case "DE":
    case "IL":
    case "KS":
    case "LA": 
    case "MN":
    case "NH":
    case "NV":
    case "NY":
    case "PA":
    case "RI":
    case "SD": 
    $("#_fid_37").val(60).prop("readonly", true);
    break;

    default: alert("Name Not Listed in any of the three cases")
}
});

Another simple choice is to make a table of State Abbreviations, and makea Relationship back to the detail records. Then pull in the numerical values on a lookup.