Technical divers will use Helium at depths where nitrogen narcosis is problematic. How much helium is added depends on the depth and the equivalent nitrogen depth - which divers usually try to limit to around 100'.
Consequently, the specific mix used depends on depth.
Some depth ranges and mixes are:
10'-100' = EAN32 or 30/30
110'-150' = 21/35 (21% O2, 35% He)
160'-200' = 18/45
210'-250' = 15/55
260'-400' = 10/70
Two things have to be considered when determining the bottom mix - the amount of helium (to keep the END around 100' to minimize narcosis and atthe same time helium cost), and the amount of oxygen (to keep the PPO2 under 1.4, or in some cases 1.2 to minimize decompression but also prevent oxygen toxicity).
For example if we are diving to 218 feet (the sand near the screws on the Oriskany), the standard mix of 15/55 would work with a PPO2 of 1.14 and an END of 79 feet. But the 15% O2 is below 16% and you'd need a travel gas to ensure you had adequate oxygen at shallow depths, especially if you are working hard swimming against a current.
Another option would be to use 18/45 which would have a PPO2 of 1.37 and an END of 105 ft. Some divers feel that a PPO2 of 1.4 or below is fine while others feel anything over 1.2 for a bottom gas is too agressive. Similarly, other divers feel an END of 100 should be a hard line, while others may feel the difference between an END of 100 and 105' is a moot issue and still others feel the END should be shallower at 60-70 ft. A third option would be to use a custom mix of 16/47 to produce a more conservative PPO2 of 1.22 and an END of 100'.
Another thing to consider is decompression. With DPlan for example with 30-85 gradient factors, a 25 minute dive at 218 ft on 18/45 would require 54 minutes of deco while 15/55 would require 60 minutes of deco for the same dive due to the lower O2 and higher He percentages in the mix and would cost more due to the higher He amounts. 16/47 would split the difference in deco at 56 minutes and in terms of cost - but is not quite as simple to aqcuire or blend.
Another area of controversy is decompression as some divers feel He mixes shoudl be used for deco such as 35/25 at the 120' stop while other divers may use 32% or 36% Nitrox instead at the same depths.
In general helium is a faster gas than nitorgen and saturates tissues faster but also off gasses faster so the deco profiles differ from air and most profiles with helium have slightly more deco and slightly deeper but shorter stop depths than air or nitrox and depth control is more critical to prevent bubble formation.
The standard mix mentioned above of 30/30 is designed for use above 100' and as such could be used by recreational divers trained in it's use. It result in an END of only 60' but frankly I think it is a massive waste of helium and it makes more sense to just do the dive on 30% or 32% Nitrox.
The helium we have on the planet was created by atomic decay over about a gazillion years, so even though He is the second most common element in the universe, it is a limited resource on earth - how limited is a matter of debate. What is not really debateable is that He prices continue to rise and will I suspect rise further to attempt to limit consumption. So open circuit trimix diving is probably going to become less common as more and more trimix divers transition to rebreathers where the He use is about 90% less than on open circuit.
