Best inexpensive connectors to pig tail aluminum to copper?

I think there are only 3 for small conductors.

Ideal #65
King Alumicon
Copalum system.

None are really cheap.

In larger wire sizes, most lugs and breakers are dual rated but there are exceptions in equipment.

It's kind of like the "Highlander;" there's only one. No need to bother worrying about 'best' or 'inexpensive.'

Alumnicon, by King.

http://www.kinginnovation.com/category-pages/products/alumiconn.php

The purple wire nuts are not really suitable for long-term use. The Canadian products are not approved for use in the USA.

Keep in mind that many boxes -especially in trailers- do not have enough room in them for both the device and the connectors.

Also be warned that many aluminum wiring jobs use an undersized (#16) ground, one that is crimped where it enters the box. As a result, the ground will often break when you remove the device, and you have nothing left to connect a new pigtail to.