In 2015 The Atlantic Monthly reported that the hiring of political crowds, (generally in cities with plenty of would-be actors), is enough of a thing to support several businesses that are open about the practice:
Crowds on Demand
A foreign government hired Crowds on Demand to help generate a
positive reception for its newly elected leader during the UN General
Assembly. The concern was ensuring that the leader was well received
by a US audience and confident for his work at the UN. We created
demonstrations of support with diverse crowds. We also used the media
primarily local and national outlets to bring more attention to these
demonstrations which led to a mostly positive portrayal. The crowds
that we deployed drew in more supporters creating a strong presence
for this leader at the UN and an improved perception of him by the
-- PROTESTS AND RALLIES, Crowds on Demand website
Crowds for Rent
These packages are created specifically for any professional organization (large corporations to small businesses), political campaigns, or any organization that needs on demand manpower to support their commercial or political objectives. We can help with the opening of a new restaurant, night club, retail store, special event, press conference or any other type of event that requires people to fill seats, lines, ask questions or write reviews to greatly enhance the profile of any business. Every business should be able to guarantee that the grand opening or a special event is a success with being reassured that people will attend. We also provide manpower for protests, rallys, campaigns, ballot measures or any other event that requires a large group of people supporting a specific cause. No event is too big or too small - we service everyone from local campaigns to national political efforts.
--Crowds for Rent Services, Crowds for Rent website
Gotham Government Relations
Not quite as open:
Gotham recognizes the inherent power found in an effectively run and targeted grassroots campaign. Employing door to door lobbying and promoting community support through consensus building, our firm will employ the most advanced strategies that have been proven successful in influencing public policy.
--Gotham Government Relations, Grassroots Campaigns website
Gotham Government Relations is too indirect to say they hire crowds, rather they organize rallies. In 2013 Salon reported that GGR recruits extras for rallies from Craigslist, and the company's parent law firm sent email to those extras exhorting them to dissimulate:
This client pays $20 an hour for a little less than an hour. All you really have to do is show up and support our rally. We ask that you DO NOT under any circumstances talk to the press or media on our behalf or discuss anything about your attendance or compensation with them.
...plus several other agencies provide similar services, that seem to be less public about it.
As of yet I've run into no numbers showing the monetary scale of these businesses, but wages are variable. It ranges from $50/head noted in the OP, to Gotham Government Relations rate of $20/hour noted above, and The Atlantic Monthly article cited $15/hour for attending events for a (failed) NYC Mayoral bid.