I wouldn't really say they are very expensive (30$ for 30 feet for example), they are active cables, to allow longer distance cable runs. They don't claim to improve the signal (a quote from the product page: "Active signal regeneration to ensure accurate reproduction over longer distances").
Not trying to defend expensive cables or anything, I hate expensive cables like "Monster" as much as the next person.
Here's a link to one of the "redmere" cables: https://www.monoprice.com/Product?c_id=102&cp_id=10255&cs_id=1025506&p_id=9430&seq=1&format=2
They look pretty cheaply made honestly
Thanks Polar Fox for attempting to derail my thread.
The question is not the expense but rather do they introduce input lag?
Does anyone have any first hand experience with this in terms of gaming?
I think up to 50 feet or so, passive hdmi cables are fine if they are well shielded, it's over 50ft there needs to be some active circuitry (some kind of amplifier to bring the signal strength back up) to keep the signal levels up.
Edit: Wikipedia says 15 meters (49 feet). https://en.wikipedia.org/wiki/HDMI#Cables
With better quality construction and materials, including 24 AWG (0.205 mm²) conductors, an HDMI cable can reach lengths of up to 15 meters