.17HMR Definitely Costs more. It's not a bank breaking huge amount compared to other ammunition but when you consider a brick of .22LR (550Rds) goes for around 20 Dollars or for the same price for .17 you get 20-50 RDS. That's quite a difference. The .17HMRs are priced somewhat similar to .22Magnum cartridges and have somewhat similar ballistics but are a little more accruate. They are much better at taking down small varmit at longer distances than a .22 LR but not much better than a .22 magnum.
That's the economics of it. If i were to give an approximation at the distances each round were to be lethal and accurate i'd say this (only an estimation not exact numbers):
.22LR good for small game 50-100yds.
.22Mag good for small game 75-130yds
.17Hmr good for small game 100-175 yds. (most accurate of the 3)
When i say good i mean the round should carry enough velocity to be fairly lethal if you hit your target.
Would i buy one? Definitely if i had extra money sitting around. What would i rather use if i were going to spend the money on a distance varmit rifle? I'd get something chambered in a .223/5.56 Nato. You get much better stopping power at 2-3 times the distance for about the nearly the same money in ammunition costs. (The rifle itself will cost a bit more though)
Just my .02 Cents
The reason you may have not heard of it is because it's a fairly new round that has only existed in the mainstream gun market since 2002 and is really just starting to gather a following.