Slightly more than 200 000 gp for a group of 20 000
For reference: this is about the same price as a +10 equivalent weapon. I would not consider this a wise investment.
A banquet is listed as a price for 10 gp per person served, and is described as such(1):
A banquet includes several food courses, good drinks, and servants to bring the food and take away empty plates. The listed price is for having a banquet at a restaurant (though some restaurant owners can be hired to serve a banquet at a private location). The price listed above is per person.
There is no adjustment stated to the price for having an banquet outside of a restaurant (which presumably does not fit 20 000 people). This comes out to 200 000 gp plus possibly an unspecified amount for catering somewhere else.
A "memorable" performance (DC 25) is listed as being able to earn 1d6 (average 3.5) gp per day(2), and has a perform DC at the high end of what low-level NPCs can hope to achieve. How much you spend total would depend on how many people can be occupied with one entertainer, but what I would consider to be a conservative estimate of 10:1 guests:performers gives only a 3% increase in cost over the banquet itself, assuming average wages.