I use mainly DVDs for storage, unless the files are small, then I use CDs. I do not use re-writables however as they are too easily accidentally over-written. The cost of blanks is so low it's not worth worrying. I tend to buy in bulk, 600-1000 discs at a time. Even DVDs only work out at <50p unless you go for double layer or blu-ray, but again that depends on saizes involved.