The show, which began in 2006, attracts 20,000-plus visitors a year and this year reached almost 24,000.
Council operational parks manager Jon Maidment said pre-sales of 10,500 for day one of the three-day Bute Park show held last week were a record. He added that the event brought £500,000 worth of economic benefit to Cardiff, which "made the show viable from a community perspective".
The organisations will split the costs. The show is budgeted to attract 22,000 visitors and would have to expand its size if it reached 25,000.
RHS shows development director Bob Sweet said: "It's been very successful. Visitor numbers have been up. We're hoping to reach 30,000."