Edit (after the first two replies below): I'm assuming a slow-clock device, say 20 MHz or less, as that's the limit of my PCB design skills anyway...
For something simple (clocks less than 200mhz), usually half a day of testing will cost between 1k and 2k I think. However, take these numbers with a grain of salt.
EDIT: they can also help you track down leaks and pass through testing the first time as well (i.e. say you're leaking tons of RF through a badly shielded ethernet port. They can track it down, and will have copper tape so you can test to see if a simple patch will fix it, which if the testing is much more expensive might be an acceptable step to add to your assembly procedure vs. doing a respin of the board).
And never assume a clean first-time pass.