I have recently noticed that on some pump and dumps Interactive Brokers’ short stock borrow costs have gotten rather high. On ECAU, SWVI, and GNIN the borrow costs have been at times about 50% APR. Even worse, Interactive Brokers does not charge based on the total value of the position, but based on the value of the position after rounding up the share price to the nearest dollar, so a 20,000 share short of a stock at $0.20 will incur borrow fees not based on the $4,000 position size but based on the amount of $20,000 (20,000 x $1). Likewise, for a stock priced at $1.06, the price would be rounded up to $2.00 for calculating position size for the purpose of interest. So to calculate your borrow cost, you need to take the APR, multiply by the number of shares and the stock price rounded up to the next dollar. That is the annual borrow cost; divide by 360 to get the daily borrow cost (which is also charged on weekends).
So for a 10,000 share short position in SWVI, with a 50% APR, the daily borrow cost is 10,000 x 0.50 x $1.00 / 360 = $13.89. For a position with a value of about $2000 that is a pretty hefty fee to pay (and the equivalent of an APR over 250%). It is still worth it for a couple days or weeks if it drops another 50% or so, but that fee is unpleasantly high. See Interactive Broker’s explanation for this policy.