I don't think cheap lockers really exist. One of the most inexpensive lockers to buy is a Detroit locker, but those aren't selectable, they are automatic which is just fine for off-road, but not too great in bad weather conditions on road. I think they run anywhere from $550-$650 each... not including installation.
An even cheaper way to go would be limited slip, but if you're going to go into the differential and change stuff, you might as well do it right and get lockers. I've seen limited slips advertised from $300-$550 each.
I have been researching lockers to death for the past few months. I am hoping to install front and rear within the year and re-gear to 4.56 or 4.88. That's something to think about too is your gear ratio. If you plan on going to bigger tires and want to get into more situations where you actually need lockers, you might as well re-gear while you're in there.
Jeeps may run on 87 octane, but the air intake seems to have a direct link to the owner's bank account.