I don’t think I would be interested in a three hour event unless it was right on my doorstep, otherwise I would be spending more time travelling and setting up than selling.
I do a regular summer event where prices start at £27 for a table or £42 for a 3m x 3m professional gazebo erected by the organisers. Footfall is about 2000 through the six hour day, and as a regular monthly event it’s good for repeat customers too. Advertising is not just social media, but includes leaflets, roadside banners, and sometimes even local radio.
For larger events such as County or Agricultural Shows I generally pay around £80 to £100 for a 3m - 4m pitch. These are long established events with lots of advertising, with a footfall of 20,000 - 30,000 over a nine hour day.
Organisers of new events tend to offer heavily discounted or even free pitches, with a percentage of sales to top up their income from a successful day.