gave them proof of over 24 years cover

I'm a member at Best med so I was trying to add my dad as my dependent, my dad had his own medical aid that he took after my mom passed away last year February, I sent them proof from May to January when my dad cancelled his medical aid to join my medical aid in February . The amount stipulated on their table was R1700 which im prepared to pay, Then when they sent the letter to me to sign the amount has hs gone up to R2800, when i asked them they said need proof that my dad was on medical aid. I took a days leave went to Bonita's got proof that he started in 1986 till 1999, then I went to KZN medical aid , i was told the company is new the old KZN medical aid does not exist anymore and they have any information on their data base for old customers. I went to the police station to do an affidavit stating what they've told me and i also stated that my dad was on that medical aid form 1999 to 2010, then i went to GEMS medical aid and they gave me proff of membership form 2010 to 2015. When i submitt all these documents to Bestmed they said the late joiner penalty is not going to change, I asked them to tell me how they calculate this persentage and they never did. I feel ive been tre