I too would say see if you can get only part of the testing since he is so young. DS6 was tested at 5.3 and it was the best thing we have done regarding his gt. It was a real eye opener not to mention qualifying scores to DYS.
Prices depend a lot on the area where you live. We paid $1,500 for WPPSI and WJ-III only and even though the psychologist dealt with gt kids she wasn't any of the known/published/highly recommended people. We live in quite expensive area though.
It was well spent money. It probably saved us a year of Montessori tuition. We might have given Montessori one more shot next year (different group and teacher, he would be on the youngest) if it wasn't for the scores.
$1,600 is really not a bad price. Just don't be surprised if you are told after the testing that you would get more accurate results at the age of 6 or 7.
