I love using trackballs on bed/couches as I don't need to worry about the surface at all. However, tablets are cutting down on usb-A ports and I don't want adapters hanging off my tablet so I'm looking for a Bluetooth trackball. It seems that the only options are the Logitech ergo mx and a Japanese trackball Nakabayashi. These are really expensive. Does anyone know if there are any cheaper alternatives?
Just buy the MX Ergo.
Unfortunately, trackballs are a fairly narrow market with not a ton of demand; unsurprisingly, this means that prices aren't exactly the most competitive. Finding something cheaper than the $50 Nakabayashi mouse with Bluetooth unfortunately is a rather tall order, and unfortunately, hackey solutions like converting a wired mouse to wireless or converting proprietary wireless signals just aren't practical—both in terms of price and the end product.
The Ergo may not be the cheapest mouse, but you're getting a solid mouse for that money. My (albeit limited) experience with their gaming and MX Master mice has been extremely favorable—something that's reflected in numerous reviews. Given how much time you're likely to spend with a mouse, IMO, it's worth getting something good.