The float class has a toString() function that will convert a float to a string. From there just parse the string so that it looks like a dollar amount.
If the parsing is giving you troble heres some simple psudo code for it:
String dollars = input.length - 2;
int loop = dollars / 3;
int stringPOS = 0;
System.out.print("$");
for ( int i = 0; i < loop; i++ ) {
for (int j=0; j< 3; j++ ) {
System.out.print(input[stringPOS];
stringPOS++;
}
System.out.print(",");
}
System.out.print("." + input[input.length-2] + input[input.length-1]);
(something like that. I suck writing code without a complier).
Java Bible ->
http://java.sun.com/j2se/1.3/docs/api/