Actually I am bored of buying costly domains like .am which are $50 for outsiders . Of course there are more domains like .tt,.im which offers two letter and one letter/digit domain which can be considered to be costlier.
.tt [src] = $500 per year. Ex: ma.tt
.im [src] = 2 letter £495, 1 letter £995 per year. Ex: ff.im
So, the cheapest one could get it for of course free . www.co.cc offers big names like sairamkunala.co.cc for free and small names like sairam.co.cc for $3 per year. And also, has a nice interface for managing the free hosting for ns, cname entries. Adding to it, you get a webstore website domain.cc.cc for free.
Note: The .cz.cc, .co.cc, cc.cc & uni.cc domains are not official TLDs. They are used for free URL redirection services, though .cz.cc, .co.cc and .cc.cc can be used as a real DNS service[src]