Microsoft sells a copy of Windows 10 Home for $ 139. However, if you do not mind buying a new free PC to take with you, you should check it out Z83II, a $ 95 ($ 95) mini PC from popular Chinese retailer Gearbest.
Now there were cheaper PCs in the past, but they usually had a massive bug. They are either used machines or suffer from inadequate memory and onboard memory (usually they have 2GB of RAM and 32GB of memory).
The Z83II is the cheapest Windows 10 PC we've found so far. It has an old, yet powerful Intel Atom X5 Z8350 quad-core CPU, 4GB DDR3L RAM and 64GB eMMC onboard memory.
It is also well connected with a variety of ports; You get a Gigabit Ethernet port (a good surprise at this price), 802.11ac (possibly an anomaly), Bluetooth 4.0, an HDMI port, a card reader, three USB ports (just a USB 3.0) and a headphone jack.
The device is absolutely tiny, barely larger than a few stacked CD cases (remember these?) And you even have a mount to attach it to the back of a monitor or TV.
It is passively cooled, which does not cause annoying fan noise when watching TV. However, you can not update it (for example, by replacing the SSD) because everything is integrated inside.
The price written on this page is true as the time it is written. It may change at any moment.