Java 7, Score = Upvotes - 3.97

Yay!!! Java!!! The world's favourite golfing language in the world. What, you can actually golf in java??? Well, it's kind of like using a bulldozer to putt.

a is expected to contain the first character. b is expected to contain the last character. c is expected to have the input string.

Here is the function golfed:

int d=0;try{d=Integer.parseInt(c);}catch(Exception e){}int l=b-a+1;char[]f=new char[l];int[]g=new int[l];int h=1;int i=1;g[0]=1;f[0]=a;int j;for(j=1;j<b-a+1;){g[j]=(h+=i);f[j]=(char)(f[j++-1]+1);i*=h==10*i?10:1;}if(d==0){h=0;for(char k:c.toCharArray()){for(j=0;j<l;j++){if(f[j]==k){h+=g[j];}}}System.out.println(h);}else{c="";for(j=l;j>0;){if(g[--j]<=d){c+=f[j];d-=g[j++];}}System.out.println(c);}

Here it is indented with structure code:

public class G{

    public static void main(String[] args){
        new G(args);
    }

    public G(String[] args){
        a = args[0].charAt(0);
        b = args[1].charAt(0);
        for (int i = 2; i < args.length; i++){
            c += args[i];
        }
        function();
    }

    char a;

    char b;

    String c = "";

    void function(){
        int d=0;
        try{
            d=Integer.parseInt(c);
        }catch(Exception e){}
        int l=b-a+1;
        char[]f=new char[l];
        int[]g=new int[l];
        int h=1;
        int i=1;
        g[0]=1;
        f[0]=a;
        int j;
        for(j=1;j<b-a+1;){
            g[j]=(h+=i);
            f[j]=(char)(f[j++-1]+1);
            i*=h==10*i?10:1;
        }
        if(d==0){
            h=0;
            for(char k:c.toCharArray()){
                for(j=0;j<l;j++){
                    if(f[j]==k){
                        h+=g[j];
                    }
                }
            }
            System.out.println(h);
        }else{
            c="";
            for(j=l;j>0;){
                if(g[--j]<=d){
                    c+=f[j];
                    d-=g[j++];
                }
            }
            System.out.println(c);
        }
    }
}

Here it is completely expanded:

public class Generator{

    public static void main(String[] args){
        beginning = args[0].charAt(0);
        end = args[1].charAt(0);
        for (int i = 2; i < args.length; i++){
            phrase += args[i];
        }
        function();
    }

    static char beginning;

    static char end;

    static String phrase = "";

    static void function(){
        int convertTo = 0;
        try{
             convertTo = Integer.parseInt(phrase);
        } catch (Exception e){}
        char[] alphabet = new char[end - beginning + 1];
        int[] values = new int[alphabet.length];
        int value = 1;
        int base = 1;
        values[0] = 1;
        alphabet[0] = beginning;
        int i;
        for (i = 1; i < values.length;){
            values[i] = (value += base);
            alphabet[i] = (char)(alphabet[i++-1]+1);
            base*=value==10*base?10:1;
        }
        if(convertTo==0){
            value = 0;
            for (char character : phrase.toCharArray()){
                for (i = 0; i < alphabet.length;i++){
                    if (alphabet[i] == character){
                        value += values[i];
                    }
                }
            }
            System.out.println(value);
        
        
        } else {
            phrase = "";
            for (i = values.length;i > 0;){
                if (values[--i] <= convertTo){
                    phrase += alphabet[i];
                    convertTo -= values[i++];
                }
            }
            System.out.println(phrase);
        
        }
    }
}