Hi Naomi,
I would start with reviewing your governing documents – does it allow subletting, and if so does it allow a fee for subletting? If yes to both questions – any fee must be applied equally to all shareholders.
The board has discretion as to what the fee will be and to increase/reduce it. Some buildings don’t allow subletting at all; some have higher fees like $1 a share per month –others a flat fee like $50 or $100. The board can set the fee as necessary under the BCL – and they don’t have to justify the decision to shareholders.

I am glad to hear your building is financially sound - Many boards can point to the increase in water bills this year along with continued rise in real estate tax, energy, insurance prices and staff costs and an increase in regulatory requirements to increase maintenance, fees etc.
Has anything else changed in the sublet policy or is it just the fees? If it’s just the fees, the board may be proactively planning financially to raise capital while still allowing sublets and maintaining costs for all shareholders.

Just my thoughts from a board member that worked hard to impose our first sublet fee where none existed, we needed to increase revenue and while sub letters were slightly disappointed – many are renting several hundred dollars above the maintenance charges - the benefits financially for the corporation have been well received by the board and shareholders.
I hope this helps.