Is liquid helium expensive because of its rarity or because of its low liquidification temperature? I once argued with my roommate about this problem. Of course it is rare. But presumably the low liquidification temperature makes the cost high too. 
So which is the primary reason? 
 A: In 2013 the cost of helium gas was around \$3 per cubic metre, and a cubic metre is around 0.18kg so that's around $17 per kg of gaseous helium.
The price of liquid helium is highly dependant on what quantity you buy, but I've seen \$7.50/litre quoted in several places. The density of liquid helium is about 0.125kg/litre, so that's \$60 per kg of liquid helium.
I have no idea how the extra \$43 per kg breaks down. The cost of liquifaction will obviously be part of the difference, but there's also the cost of storage and distribution - you can't just put liquid helium in the post.
A: Cost is determined by supply and demand, and I am tempted to suggest moving this to some economics (stackexchange?) site. For Helium, our defacto only supply is fossil fuels as nothing else seems to contain any concentrations worth mentioning. Because of its potential strategic importance (for 1920s airships and 1950s rocket technology), the USA is maintaing a strategic reserve, the National Helium Reserve. The back and forth over its future has contributed to price changes in Helium over the past two decades, but I do not have actual numbers on that.
Considering that we will run out of any significant sources of Helium once we run out of fossil fuels but will almost certainly not run out of applications for it (think superconductor applications and low temperature research), one can, at least qualitatively, predict high prices. For anything more quantitative or nuanced, may I again suggest turning to economics rather than physics?
A: A significant factor in this is that helium is hard to produce. There is very little helium in the atmosphere because it is so light that individual molecules have a good chance of achieving escape velocity at atmospheric temperatures so it disappears into space fairly quickly. 
Also because it is very uncreative it doesn't get bound as chemical compounds, unlike hydrogen which is straightforward to extract industrially from a variety of sources including water. 
The main source of helium on earth is as a byproduct of natural gas extraction where it is trapped in natural gas wells as a result of natural radioactive decay. The national helium reserve in the US has already been mentioned in another answer. This was a strategic reserve at one point in the 1920s-30s when it looked like helium airships would be of military importance but when heavier than air aircraft overtook airships in development there weren't that many applications for it as a  bulk commodity. 
However that changed with the advent of applications for cryogenic superconductors, especially in medical scanners and other scientific instruments especially where very powerful magnets are required and there is an argument, in hindsight,  that a lot of this finite reserve was effectively wasted in by selling it on at a very low price. 
Other industrial applications of helium include shielding gas for specialist welding applications and as an inert alternative to nitrogen in SCUBA breathing gas to alleviate the problems associated with nitrogen decompression/narcosis, particularly for deep and/or long duration dives. 
By comparison other industrial noble gasses which can be harvested from the atmosphere (despite having very low concentrations) are relatively cheap. For example argon is the main constituent in most gas shield welding applications and reasonably affordable for even hobby users. 
