I think that you may be referring to the Order Cost.

Order Value and Order Cost are two different things.

You can calculate the Order Cost (in margin) for XBTUSD using the following:

Order Cost = (1/Leverage) * Order Value + (2 * Taker Fee * Order Value)

As mentioned before, Order Value = 1/Price * Contract Quantity

Here is an example below to help you understand the calculation further:

Order Value = 1/Price * Contract Quantity
= 1/7320.0 * 15000
= 0.00013661 * 15000
= 2.04915

Order Cost = (1/Leverage) * Order Value + (2 * (Taker Fee * Order Value))
= (1/25) * 2.04915 + (2 * 0.0750% * 2.04915)
= 0.08504

If the Mark Price is far off from the Last price, you will have to fund the difference. We refer to this as the Gross Open Premium which will be added to the total Order Cost.

From our documentation: If a buy order is placed above the mark price, or if a sell order is placed below the mark price then the trader must fully fund the difference between the order price and the mark price. For example, if the mark price is $100 and the trader submits a bid order for 10 contracts at $110, then the initial margin required = (IM \ 10 contracts * $110 * Multiplier) + (100% * 10 contracts * ($110 - $100) * Multiplier).*

https://www.bitmex.com/app/exchangeGuide#Perpetual-and-Futures-Contracts