You can't upload sketches to the serial port if you don't have a bootloader installed.
If the USB-to-Serial chip you have is an FTDI chip like on the Arduino Diecimila or Duemilanove then you might be able to use the FTDI chip as a "bitbang" serial programmer: Arduino Playground - AvrdudeFTDIBitbang
It's going to be hard to produce a programmer that can sell for less than $4 like the USBasp programmers available from HongKong. Yes, that $4 includes shipping to the USA: