Solid core power cords under 300$ - List of manufacturers ?

That pesky liability! Say'n!

The reason there is an aftermarket of power cords is precisely because solid core wire is illegal for use in a power cord. Romex is extremely high performance, which is why you really don't have to worry about what's in the walls (if that is what you have), and why that last 6' of power cord can make a difference (because it has to be multi-stranded).

But the liability of making a power cord out of romex is profound, so no-one actually does it.

So, in short (pun intended) there is no such thing as a power cord made of solid core wire.

Solid core AC power cords can't be UL approved as they can be dangerous for a number of reasons.