You can't find any laptop that runs CAD well under $300.
That being said, there are laptops that will be able to run it decently enough for it to be usable.
If you're willing to go a bit over budget ($339), this should suit your needs.
https://www.amazon.com/HP-generation...PC94QE1A258BEE
I used to use an older Pavilion model (which are worth taking a look at) that has a slightly weaker CPU and graphics card than the one in the link posted, and I could run Autodesk Inventor well enough to do whatever was needed. Even after messing with all the settings I could to lower the resources the program needed, it was still pretty laggy, but not enough to be a massive issue.