Price increase with R 1 500 in 2 month

So in January I got a qoute for 30 people from OR Tambo to Cape Town, the qoute was roughly R 900 pp for a round trip. I informed them that I was just looking for prices to budget and was aware that the prices would increase. In March I asked for a new qoute for 50 people and the new price was R 2 500 pp for the same trip. On query of the price increase the response I received \prices are subject to availability"how does the availability of the seats push the ticket pricexsup with R 1 600???? A booking for a non group flight the prices for a round trip to Cape Town start at R 690??? That is a big leap from R 690 to R 2 500?? """