Welcome to the nightmare of A38(1)(c) - "excessive demand medical inadmissibility". You have three options at this point:
(1) You can ignore the fairness letter; they will refuse your application.
(2) You can hire an attorney to help you compose a response and lay the groundwork for a legal challenge should they refuse you. This option will cost between $3-5k most likely.
(3) You can educate yourself on the process and prepare a response to the fairness letter. If so, you should expect to spend 50-100 hours reading, learning how the process works, and preparing your response. There may still be some costs associated with this.
As I recall, the threshold is $6285, though it is due to be revised (that value is rather old). You are fortunate that they provided you with a cost estimate.
First, you should confirm that this is the actual cost of the medication. I've found that CIC often does not have accurate cost information. The best source of this I've found is actually the Quebec formulary, which lists the prices that Quebec pays for such drugs.
A fairness response can take several approaches:
* You can challenge the medical diagnosis. This could include their claim you have the condition as well as their estimate of the treatment and/or costs required.
* You can present a mitigation plan. This would be where you present evidence of insurance. Of course, it is far more convincing if you show that your insurance is already actively paying for the drugs rather than them being provided by the province. As part of a mitigation plan you should include a declaration of ability and intent. There is a sample of this in OP 15 (from the CIC website) as I recall.
* You can set up arguments that would provide fodder for challenging a refusal. For example, it is best practice to request a TRP to overcome the inadmissibility (there's a $200 per person fee for that application and there's no formal form for it - you have to explain why you should be granted a TRP). While a TRP won't give you PR, after three years you can apply for PR in the "permit holder's class" and they cannot refuse you on the grounds for which the TRP was granted. Other arguments relate to the violation of Charter rights (you have a disability) and the obligations that Canada has undertaken as a signatory to the UN Convention on the rights of people with disabilities. There's an argument that the entire section is itself ultra vires (which means the Canadian Federal Government has no right to make such decisions) because health care belongs to the provinces exclusively. I've also argued that the very mathematical basis is suspect: they use the average Canadian costs, but they use the maximum costs in your group - because you will BOTH be deemed inadmissible. Thus, the argument is that the excessive demand threshold is really $6285 times the number of people in the application. In your case that isn't sufficient anyway, but these are the kinds of arguments that can be included.
Most important: don't panic. If you aren't comfortable doing this on you own get an attorney. This is a HIGHLY specialized area though and there are only two that I can directly recommend because both have won significant decisions in Federal Court in this area (CIC doesn't do so well in Federal Court on cases similar to yours).