My laptop runs AutoCAD and various Autodesk drafting/modeling programs. I'd like to buy a second monitor to help me work with multiple files/drawings at the same time.
What second monitor can I buy that...
The Asus MB168B+ looks like it would fit your needs. It:
is designed as a second monitor with a USB 3.0 cable to power it, and transfer video and data (also works on USB 2.0, but you may experience worse results video quality-wise)
has a 15.6" screen with 1920x1080 resolution
is very portable, weighing less than 2 pounds, and being about as thin as a pencil (0.3 inches); it also comes with a case for protection and also to act as a stand
can cost under $150; prices range from about $140 to $225, depending on where you buy it (bargain stores are cheaper); if cost is your biggest issue, you may want to check out the Asus MB168B (no "+"), which is very similar except that it has lower resolution (1366x768) and overall lower quality display; it can typically be found under $150 at well known stores
If you prefer higher quality display over cost, the MB168B+ is the better fit, but if you really need it to be under $150, you'll probably want to go with the MB168B.
There are a few more factors than the ones you listed that will really play a part in how much you'll have to pay. One being the panel type and two being the input type(s).
IPS panels (the screens that feel like glass) cost more but have a better viewing angle, more vivid colors, and they're usually a little more sturdy. TN panels (the screens that make weird colors if you press on them) cost less and are more common but are usually faster in terms of response time. Thus, I'm thinking you could use an IPS if there's a good price.
Monitors aren't really ever made to specifically be a primary or secondary monitor — they're all the same in that sense. So that's out of the question.
Portability is very subjective. One person might think a monitor is portable while another might not. Personally, I judge portability by the box the monitor comes in. For example, my Asus VE358 comes in a box that can be used as a big briefcase. Other manufacturers might not do this.
As for input types, you just need to know what inputs your laptop has. DisplayPort and HDMI will cost more than VGA because they carry audio as well as video.
So, on to picking some monitors out. I have one that is on the high end of your budget and a few on the low end.
In terms of sturdiness, I lean towards Dell because they use a stronger plastic for most of their mid-tier monitors as opposed to something like Asus where the plastic is a little bendable.
