Usually a WISC is $500-1000. At our local community college, they test with the WISC etc for only $75. You don't get to choose the test and it is with a psychology graduate student. Still... that could be cheaper.
On the other hand, sometimes you get what you pay for. For some of these expensive testing sessions, you may get a very kind professional who knows how to test gifted kids or deal with the child's idiosyncracies. They will usually let your child have breaks if they need it.