Not all raid is that expensive (though some is very expensive). The basic concept is pretty easy to understand. Raid 1 and raid 0 are the most common on home computers, and in fact, most mother boards support them with no additional hardware (except the hard drives of course). raid controller cards are cheap too (starting at about 13$ though more for a good one). for external, there are external 2 drive raid enclosures starting at 43$ on newegg (probably a piece of crap though), but several under 100$. It is generally best to have identical hard drives (not only same size but same exact model, and if possible, same revision etc).
common types of raid
This is not for backup, its for speed. I only mention it so you are not wondering what it is. It basically splits your data up into little chunks (often 4-64k) and sends them alternately to both drives. as you are reading and or writing to both drives at the same time, you can move a lot more data in the same time frame.
basic mirroring. duplicates of your data are sent to both drives at the same time. you basically have identical copy’s of your data on both drives. The big disadvantage of course, you loose half your space. you are writing the same thing to both drives. You can even use a raid 1 drive as a boot drive (and if 1 drive fails, your system doesn’t even stop working, it keeps running on the good drive till you replace the bad drive, and rebuild the raid array). It only protects against a single drive failure though. If a virus or other software related issues corrupt your data, both drives will probably get corrupted. IF both drives die (lightning strike, power surge, cheap power supply falure), you of course loose your data.
I haven’t ever used raid 5 but basically, it uses parity data on all the drives, so that if any one drive fails, you can recreate the lost data from the other drives. It requires 3 drives minimum, and on a 3 drive raid, if I am not mistaken, wastes 1 drives worth of space on parity data (so you loose 1/3 your space). IF you loose 2 drives you loose all the data. Raid 5 can work with more than 3 drives but I’m not sure on the particulars. The same concepts of how it works to use parity data to recreate lost data on a lost drive still apply though.
Raid cards for internal raid 5 are not very expensive (better ones of course cost more). External raid 5 is where things seem to get VERY expensive. This one is only about 500$ (with 4 500 gig drives), but it doesn’t seem to be true external raid.
The true external ones are much more. Fyi, I have seen a few that are saying 1.5GB usable on a 2GB 4 drive raid 5.
Raid 6, similar to raid 5 but allows 2 drives to fail at the same time. If I’m not mistaken it takes even more drives and is even more expensive.
Hope that info helps some. Fyi if you want to go cheap, high capacity, reliable backup (but slow), maybe you should look into tape? Back in the day it use to be good cheap backup and pretty safe as long as you don’t get a magnet near it. I’m not sure about cost of it these days though (its still out there but not quite as main stream as it used to be).