I can't say for sure since I am not familar with the underhood layout of a Kia. However, I have replaced several thermostats in my own cars over the last 60 years and $300 would have been a rip. Thermostats with gasket is less than $5; the remainder of the cost is labor and having tools to do the job.

Coolant is lost when replacing a thermstat. How much is lost is determined by where the thermostat is located. In most cases, it is near the top of the engine and loss of coolant is minimal. However, at minimum, the cooling system must be topped off and burped of air. Refilling and cycling the air out of the system takes more time than the actual changeout of the thermostat.

Usually, I drain the coolant system and replace the coolant when I change a thermostat. I'd allow a shop 1/2 hr to change the thermostat plus time for refilling the system. My do-it-yourself costs are listed below:

Thenmostat & gasket $5.00
Antifreeze, non-silicate $10 to $20
Water pump lub $3.00

This is $18 to $28 of parts and supplies.
I can take the old coolant to the village recycling tsnk at no charge. At a dealer or garage, there will be a disposal charge - cost varies, but $5 should cover it.

Labor/time at $80 per hour at a dealership is the largest part of the cost.

At $300, I'd shop around. He should be able to find an independent garage to do the job for less.

I will say changing the thing did require a bit more work than changing one on a domestic car but nothing to warrant 300 Dollars. The biggest problem was finding the correct wrenches, Marcel has been retired several years and his tools were not being used so finding the right ones required some looking. However, we made it, had a lot of fun recalling old times and cars, plus we helped a neighbor in need, made us old codgers feel useful, the youngest in group is 73 and oldest me is 86. Iggie

That's good to hear! Good for you and your buddies. That's how we try to do things around here, too, everyone helps each other unless/until nobody knows for "free"...

My brother couldn't get the back door of his Explorer to open although we could hear the electronics functioning inside the latch. DH ended up taking off the inside panel and discovered the wires that operate the latch had slipped out of place because the rubber things holding them on had rotted. While he was out there looking at it, we were inside joking that he'd probably rig something with some duct tape and a coat hanger. Sure enough, when I went to see if he needed anything, he sent me inside for some duct tape and a coat hanger. It's working now. Good times!

depends where thermostat is. i think on some cars it is buried down by waterpump which maybe under timing belt cover. new cars with overhead cam motors are notorious for expensive timing belt jobs. you replace belt, water pump, tensioners and other stuff. i do know some cars have thermstat in really awkward places.