AES key storage for a C# application - USB HSM solution

AES key storage for a C# application - USB HSM solution

I am working on a C# server application and one of the requirement is to
encrypt data using AES. The key must not be stored on the server but on a
secured USB hardware device (to be easily transportable and easily removed
from the server). This application is used only few days a year, that's
why keys should be stored on a portable device, in order to be removed
from the server and securely stored, in a vault for example...
After some online research and on stack overflow I heard about USB
"hardware security modules" (HSM), i found the YubiHSM who seems to have
great features but is over my budget. Did you already use such solution ?
How difficult it was to integrate it with a .Net application ? As often,
budget is a key constraint... I am looking for a device with a cost < 200
USD.
In short, here is what i am looking for:
USB device with the ability to securely store AES keys
.Net Integration (SDK?)
Cost < 200 USD
I am not sure a such device exists, especially at this cost, then please
don't hesitate to share your experience!
Sorry for my poor English and thanks for your help :)
Doots