My 2 yr old daughter just had a renal ultrasound last month. Not at all painful, just like the ultrasounds you had when pregnant. I noticed in one of the other responses that the Mama talked about a valve in the urinary system not functioning properly. This will not show up in an ultrasound. If there is a valve problem you need a procedure called a VCUG (voiding cystourethrogram)which is an x-ray procedure where a contrast fluid is injected via catheter into the bladder and then peed out while x-ray pictures are being taken. This procedure is mildly uncomfortable just because of the pressure on the full bladder, but once the baby pees the pressure is gone and all is well. This procedure checks urinary system function, an ultrasound can only see if there is a problem with the shape or size of the anatomy, not if it is functioning properly. I would ask your doctor if you should do both of these procedures, or why he want to only do the ultrasound.
As far as cost goes, I just got my bill from UNM total cost for both of the above procedures, $1248! Check with your insurance to see how much they will cover. My daughter had this done because she has recurrent UTIs, she even got one while on antibiotics for another one. If your daughter has only had one you may want to hold off as another mama suggested.