The question that begs is: does he care if he loses all his data if a drive faults (more space cheaper), or does he want it backed up (less space, unless you dump $$ into it) safely?
I want it backed up, I'm not expecting my collection of things I want stored on it to grow super huge, I'd be very surprised if it grew past 1TB. However, it would be nice to have it all backed up. This is actually something I have put much thought into the best way to do this and one of the main reasons for making a RAID. The problem and the thing I am very concerned about is multiple disk failures occuring at/around the same time. I seem to come across a fair number of stories where events like this have happened.
For that reason I have considered other backup methods but none of them appear all that promising. A tape drive will set you back around $300 and then the media for that is even more expensive. DVDs are probably the best option but how long they will last is still in question as well as the fact that at 4.7GB it would take a lot of them to backup a 1TB collection for example(somewhere over 200 I believe). So that doesn't bold really well and if you have to store that many you lose the advantage of being able to store a copy offsite with ease. The only good thing about this would be the media would only set you back someplace in the $60-70 range so thats not horribly bad. But again the numbers are killer since it will take a lot of tedious time when say Bluray or HD-DVD comes around to migrate a collection over to those.
Right now my photo collection stands at about 30GB or so. So its not incredibly huge or such. But the main reason for this #1 is I haven't been taking as many pictures as I could and #2 I have been learning so I didn't do things correctly for the first 20GB or so of pictures. #3 I actually edit only a small amount now because the Tiff files are rather large, 50MB Tiff file while the raw is 7MB and I want to save both the Tiff and raw file. I'm also considering adding a (HD)video camera to my collection so that will need even more space. Basically I can see the need for lots of space and a way to ensure that I am able to keep it backed up for a long time to come.