I just found some interesting info about RAID’s. It appears to be that almost all cheap (below $300) RAID controllers are so called fakeraids. Meaning they don’t have any RAID hardware onboard. They are just plain PATA/SATA disk controllers that have no RAID features plus some proprietary software that does all the RAID-related calculations. Because the software is proprietary, it is usually hard to make these things work in Linux.
My friend has HighPoint Rocket Raid 1640. He paid around $100 for that, which was more than some other offerings. He believed he is getting something better by paying more, but it appears it was not the case at all.
Real RAID controllers usually have their own processors (some are old and slow but some are modern and fast) and (upgradeable) memory, which makes them expensive.
The conclusion is that if you want a cheap RAID in Linux, just use plain PATA/SATA controllers and open-source software RAID. If you want real hardware RAID, prepare to pay several hundred dollars.