NAPA has the proper Zerex G5 coolant and it's under $20 per gallon and as I recall it takes just under 2 but I could be wrong on that.
When my thermostat needed replaced, I pay a $100 deductable on my extended warranty, I asked since it was about 5 years old how much to change the antifreez fluid. He gave me a off the cuff $500 quote
I ended up replacing the thermostat, just over $100, 2 jugs of G5, just under $40, and 2 jugs of distilled water that cost $2.
So for under $150 I did the job in about the time it would have taken me to make two trips to the dealer at a cost of$600
$100 deductable and a $500 flush 
For the flush with the thermostat off I ran tap water through all the hoses then used air to blow any remaining water out. It ended up taking the exact amount of 50/50 coolant listed in the FSM
