What a margin loan costs, broker by broker
Interest on a margin loan is the balance times the annual rate, accrued daily. At published rates, $50,000 borrowed for a year costs from $2,525 at 5.05% to $5,850 at 11.70%.
What will borrowing cost?
Published rates, simple interest
What the calculator leaves out
Rates float. Most schedules are a base rate plus or minus a spread, and brokers move the base rate when the Federal Reserve moves its target, so a 12-month figure is an estimate. Interest you do not pay is added to the loan, which then compounds. And the rate printed on a schedule is not always the rate a large account pays: several brokers say to call for balances of $500,000 or more.
Margin interest may be deductible as investment interest expense if you itemize, up to your net investment income, using IRS Form 4952. That is a tax question for your own situation, not something this page can settle.
Sources: each broker's published schedule (linked on its page); SEC, Interested in Margin? Understand Interest; IRS Form 4952.