We can do some calculations. The average Class A office space rent in midtown Manhattan was
about $75/sq ft per year in Q1 of 2015. No doubt Trump Tower is Class A. In fact, it's probably on the upper end of Class A. So, let's assume that it's $85/sq ft per year now.
The office "rule of thumb" allocates
125 to 225 sq ft per employee. For campaigns, it's probably towards the low end, so let's assume 125 sq ft per employee.
Trump's campaign had 172 employees and consultants in the office in July. That means at least 172 * 125 sq ft = 21,500 sq ft of office space. At the market rate of $85/sq ft per year * 1 year/12 mths, you get 21,500 * $85/12 =
$152,292 per month.
Seems like the rent is in the market ballpark now, and that the rent paid back in March was significantly subsidized.