Table Formula: Update column, then update value

[torys.anderson@gmail.com (Tory S. Anderson)]

I have a table that calculates daily cost and then sums the hours and the weekly cost. But I can't seem to get a formula that will do the former.

  | Date         | Hours | Rate | Cost |
  |--------------+-------+------+------|
  | September 8  |  0.50 |    1 |   1. |
  | September 9  |  2.00 |    1 |   2. |
  | September 10 |  2.00 |    1 |   2. |
  | September 11 |  2.00 |    1 |   2. |
  | September 12 |  2.00 |    1 |   2. |
  |--------------+-------+------+------|
  |              |   8.5 |      | 399. |
  | ^            |   hrs |      |  ttl |
  #+TBLFM: $hrs=vsum(@I..II)::$ttl=vsum(@I..II)

Before the formula above works, I end up pasting the following line just below the last date (above the @II line): 

  #+TBLFM: $4=vsum($2*$3)

So, I have to copy-paste into the middle of my table, run the line, then cut the line back out of the table and run the lower line. I'm just sure there's a better way; it even seems like once upon a time I had something working. But is there a non-repetitive way to autofill the COST ($4) column between @I..II without wiping out everything else?