Firefighter, We really need more info to go on in order to help you out.
1-By portable, do you mean something that you can toss in the back of a truck or something that you tow behind the truck?
2-How many people are you planning on feeding at a time?
3-When you say you want to use wood, what kind of wood? chips, chunks or splits.
4-In any unit that you can burn wood, you can also use charcoal. In my Lang, I primaraily use wood splits but I have used Lump Charcoal on a couple of occaions.
5-And the BIG questions is -What is your budget.
As for cheapest, it's a toss up between the ElCheapo type smokers (also known as Water Smoker for the big ol water pan they use and a build yourself Ugly Drum Smoker.