10.4 is $199 for the Family Pack, which is a license to install it on five machines. Just one copy for one machine is $129, and if you're a student or work for a school, it's just $99 with the educational discount.
Mac OS X 10.5 (Leopard) comes out in October and the prices will be exactly the same, so if you can only afford one upgrade you may want to wait until then.