Don't you worry. With the number of them that Trump buys, I'm sure he has negotiated a sweet deal for them. He probably gets them for only $2K.
Let me try again. I misinterpreted "bills" as being $1K rather than the normal $100. I actually went to the linked page, and found the price of the pen to be $374.50, which is close to the 4 bills you mentioned. Yes, that's expensive for a pen, but not what I was mistakenly thinking.
I bet they are a hot seller these days.