they are very hard to breed PERFACT water and great care is the only way. You cannot go out and buy just 1 discus you need at least 6 jouvaniles (small discus) they are shoaling fish they need alot of water changes and a good size tank.
I would say they are so expensive is because you are looking at the rare and most colourfull ones you can get a snow white for 15 pounds you will be looking at pigion bloods they have alot of colour thats why they will be expensive there is also the malboro red they can cost 100+ poundfs as these are rare and have great colour to them. I think you should go out and revise about these fish before you buy them ok.
also they guy who said you NEED ro this is not true i use tap water and my discus are fine they will just not breed and i am happy about this because they are PETS and thats what people dont think they buy discus and just think oh great a lovely breeding pair will soon come out and 150+ punds will be in my pocket but i want them for the lovelyness and the happyness they do not for the money.
so if you dont want to buy a ro you dont have to ok B)
DM
a few tips
PH 6.5 to 7
nitrate 0
ammonia 0
nitrite 0
I keep mine in a juwel rio 180 and i think that is the smallest you should get.
Get a good external (ehiem)
No hard currents
not alot of lighting (lots of shady patches in the tank so htye can hide)
choose tank mates carfully
look for the roundness of the discus when you buy them dont go for odly shaped ones always round
go for a discus that has no disease you dont want your 50 pund fish dying on you do you?
Make shure the tank can hold dinner plates as they grow to the size of them.
Hope this small advice helps you there is alot more but i would be writing all day so go on discus fish UK to get some more help.
DM