It's not even close to the least expensive way - various Chinese-made USB-to-serial chips are cheaper, as is the Atmega8u2 used on more recent Arduinos. The Atmega8u2 is a fully-programmable microcontroller too, as are the larger chips in the range.
You don't even need that. The attiny85 is sufficient.
See https://github.com/Bluebie/micronucleus-t85 It's a bootloader that detects usb in the boot process. If usb is hooked up, you can upload programs, then it will run whatever code is loaded. This bootloader is the same one used on the digistump.
You need 3 diodes and 2 resistors along with the attiny85 chip to make your own usb to serial converter.
