If you're looking for the cheapest Darkrai variant, just plain Darkrai with Crushing Hammers is probably the cheapest, and one of the best. Crushing Hammers are really cheap, and the rest of the decks space is just trainers basically. If there are any Item cards that you think would be useful, but other decks just don't have the space for, you can probably fit it into straight Darkrai.
If you're looking for a list, just search around the Decklist help section, you should be able to get the gist of the deck from the lists people post. Zarco pretty much listed all the variants, except for Darkrai/Dusknoir, and also Darkrai/Keldeo is really just straight Darkrai with Keldeo as a 1-off tech.