Just got back from havin' to have youngest DDs SUV towed. She had to park it in a Walgreens parking lot when it started to overheat on the road. Luckily, she keeps an eye on gauges.
The overflow tank was empty, so I topped it off with coolant, but the coolant didn't go down and start to circulate, and it started to heat up again. Most likely the thermostat.
Looked online while waiting for the tow truck, and the part is only about $40. Labor will probably be somewhere in the $200-$300 range, though...it's definitely not as easy to get to or replace as the one on my '71 Olds Cutlass...