I am looking for a simple USB monitor that is between 11-15 inches screen size for a cheap price. I'm looking mainly for portability that I can plug into a laptop to increase productivity. I would also need the monitor to be as thin as possible. I would be fine with a HDMI monitor too but I'm thinking of getting a couple and I only have one HDMI cable. Screen rez isn't a concern for me. Only as long as its not lower than 720p.
ASUS has a series of 15" mobile screens (with foldable stands similar to iPads) named MB167B, MB168B (2nd generation), MB169B (3rd generation) and MB169C (with USB C instead of USB A/B connectors). Additionally there are also models which end in B+ and C+ which are FullHD instead of just HD. AFAIK they're all USB 3, but also work if only connected with USB 2 (then only with reduced brightness/backlight). Not sure if you consider them cheap, but at least I don't consider them expensive.
You didn't specify the operating system you're using. So one more comment on these screens: One drawback is the lack of proper Linux drivers. There seem to be pre-compiled drivers for Ubuntu, but you'll find customer reviews that they don't work properly even on Ubuntu.
(That's the main reason why I never bought one. The other one is that 15" is already too big for my tastes. So I'm curious if other answers show up to this question. :-)