It's tax and shipping costs essentially, I noticed the price didn't go down at all when the VAT was dropped which really irked me.
Basically it's what's termed a luxury product and because it's from the far east it is already coming in at a premium as such we get charged with premium pricing. A lot of it is name snobbery, there's really no denying a lot of the product is good and this is from a cynical ADA user (a rare thing), there's also no denying that a lot of the stuff they peddle, is pseudo science and completely unneeded in any capacity.
The main thing is that ADA have lost ground, Aquasoil was their biggest spinner but now there's columbo and Olive knott's both of which have the same properties but without the issues such as yellow water and leaching. The price issue is still there because they're small companies and still hit with non essential items tax levels.
Their lighting is pretty but there's very little to differentiate between say, aquamedics £150 halide and one of their £600 halides, some PAR diagrams I've seen on reef forums, using the same bulb, actually suggest the distribution from the AM reflectors is better. The bulbs are very good however but aren't to everyones taste.
Their ferts are well packaged and good, but now that Tom barr has a detailed analasys on them you can make your own for a fraction of the cost and diy dose using the same guidelines, It's essentially a very low dosing ppmd I'm lead to believe.
The final thing ADA are known for is their glass wear. I've got an ada beetle and I can honestly say that my rhinnox 1000 pair are supperior in producing a fine mist for a long period. Yes, I've had bad ones but at £4-6 who cares. Even the ADA ones don't give "whole disk diffusion", that's something that is near to impossible because of the nature of ceramics and pressure, it's near impossible to produce something where exactly the same pressure is needed everywhere on it's surface to permiate it, gas just like water or people will take the easiest route where ever possible and that's normally over a small portion of the disk.
The lily pipes are a contentious issue, people who've used them will never go back, same with people who've used Cal Aqua, both are roughly the same price these days (when I first bought my cal aqua nano pipes direct from cal aqua they cost me $25-£17 including postage, god knows what the mark up must be now they're nearly £50!) it's a person preference thing. Do aquas are supposedly good but all glass wear is essentially a copy of the original ADA stuff, and as such all could be improved to increase through flow and reduce friction and reduced flow rate. The PM ones are very good, ANS's are better (much stronger/thicker glass) but if you want ADA you'll always say ADA are better. The inflows are what let down all but CAL Aquas pipes (it does but much less) the slits are too few, too small and poorly distributed. Do Aqua seem to have addressed this giving loads of inlets, widely spaced but from what I can tell only on their nano range of pipes.
Even then recent experimentation and annecdotal evidence has shown that standard outlets/inlets might actually be superior in producing good flow throught the whole tank without pressuring the filter reducing flow. A spray bar is almost always going to be the best for achieving even flow but they're unsightly so there's the trade off which means we choose lilys, the market is ready for some experiment to produce some glassware, even plastic glass wear that's clear, that is capable of producing even flow, doesn't reduce flow and is most important, doesn't really detract from the scape.
I've been following threads meant for reefs and Loc line is the item of choice because of it's modular aproach and that you can aim the water exactly where it's needed. Something liek that but much less intrusive would be a god send.
In summary, yes ada is expensive and you're paying a lot of the time for the name, but some of their products are worth it, (just as a fair few aren't) you're also paying shipping and tax much higher than normally associated with an item. Yes you can get the same thing cheaper, emulate their products through DIY and get superior products but just like apple you buy an experience, or an ideology and it's pretty, conveinent and easy to follow has an air of superiority and because it's a product thats image is seen as being luxury it prices itself as such because people will pay it if the aesthetics and results match the hype and are convenient for the person to follow. Just like building a pc that is more powerful than a mac and is cheaper it's going to be ugly and have linux or windows both of which aren't particularly nice to look at. A mac will cost more, and be less powerful but it's a mac and it's prettier to look at and prettier to use and has the air of superiority and as such people flock to mac.