That question basically puts it all into perspective. Values are all about who needs which carb and for what car. As mentioned above in another post, generally speaking the 66-67 carbs (while certainly not cheap) are the least valuable, 70-71's are the most.

What complicates it are the variables (with any year).

#1. Which date codes the carbs are.
#2. The SPD of the car that needs them.
#3. Which transmission the car has (68 and up, 66 & 67 are all the same)
#4. If buying/selling singles or pairs (date matched pairs usually bring a premium)
#5. Do they both have original tags?
#6. Are they complete and unmodified?
#7. Are they restored or just cores?

All of the above are contributors to Hemi carburetor values. Are they overly rare? By production counts, not overly so, but many have been lost, modified, or put back on cars over the last 50 years since they were made, every day there are fewer available. So, if you consider all of the above, the set that is perfectly matched for whichever car can prove to be elusive and is usually going to be costly.

I occasionally take in cores in trade for work and sometimes buy mates for singles I pick up etc. and sell them in restored condition If the prices of originals is beyond your budget I sell "clones" which look very similar, use all original factory linkage, fuel lines, air cleaners, etc. for $1,200.00. Finding good carburetors these days is a lot harder than it once was, you really have to be careful as there is some real junk out there and then there are the re-stamped fakes (ebay is a mine field) . So, be careful out there buyers.

.02