You should know that the Source of Earth Special Forces has been made for Half-Life, making it stand alone would require a re-code due they have to do everything over again. As said above by David. Half-Life is not even that expansive, it's rather cheap if you go look at the other games that are over 30 to $90.
ESF was built on the HL1 engine, and like Sky said, they can't make it standalone without having to do heavy recoding. They'd still be using the Half-Life engine, but it would be illegal, because then people could download ESF for free and use the HL engine for free.