the demand of Lv5 trainer is too high
Perhaps the BBs have purposefully allowed the prices for trainers to increase. The market is out of control in general right now and people have too much money to spend. Bonus costs are a way to take money out of the market.
If you look at it another way, players fetch more value on the transfer market now, so the money made from selling trained players has gone up. In order to keep training profits constant, you have to allow the prices for trainers to increase, otherwise the profit margin for training gets proportionally too high compared to other profit margins.
Run of the Mill Canadian Manager