I use Grab almost every day and it has never jacked up the price after you book it. Before you book, yes sometimes it has during busy period but on the other hand it has also reduced the price. The meter taxis when they see a foreigner tries to get a fixed rate. If you know where you are going and roughly the price and depending on the time of the day you do occasionally get a better deal. To and from the airport, what they normally try to do it to get you pay the highway toll and in this way they save time.
The fixed 500 rate is the going rate from CBD ( Central BKK District) to the airport by private cars. This should include the highway charges too.
In my experience Bolt has a lot more cancellations and less vehicles than Grab. But with Grab at least you know how much you have to pay unlike a metered taxis where they can take you for a few extra few miles.