The only things I have actually bought for the R-Pi are:
1) A Micro-USB cable, which Nokia wanted €29 for - I told them where they could stick it and found one for €7 instead at the local equivalent of Tandy/Radio Shack. This means that I can loopback-power the R-Pi from a powered USB hub.
2) Entirely too many SD cards, all different. Some are definitely better value than others.
Everything else that I would need to run it, I already have in spades - I could quite literally build several PCs just from my spare parts piles. But some tangentially related purchases:
3) A USB3 based card reader, so that I can see if the cards are capable of more than my TrimSlice will drive them to. I only have one PC that would be able to drive it properly, but it might have higher performance than a standard card reader even on USB2.
4) A replacement for about half my home network infrastructure, in the form of a new ADSL modem and an extra 8-port GigE switch, plus a significant amount of Cat6 cable. This allows me to hardwire an extra computer desk instead of relying on wireless - both the modem and some of the wireless adapters were proving to be rather unreliable.
Interesting contents of my spares pile, which might get incorporated (at least temporarily) into my R-Pi setup:
5) A very small (maybe 9") monochrome CRT monitor, which has a composite input. When I remember which box it's buried in and unearth it, the date on the electrical safety tag should be amusing. Excellent for checking whether a given display remains readable on an old TV.
6) An IBM Model M keyboard - I have two, though one usually lives at the office. According to their birth certificates, they are both old enough to buy alcohol. It's even possible that I might turn one of these into a *case* for an R-Pi, especially since a faulty cable suggests that a conversion to USB would be a particularly good idea.
7) Several sealed lead-acid batteries, together with charging equipment and at least one 12V-to-USB adapter - thus making a relatively effective UPS. Once upon a time, I used such a battery and a pair of actively-heatsinked 5V regulators to jury-rig a CD-ROM drive which I could use on a train journey - this being before laptops were readily available with built-in drives. I only did this because the CD in question would only be available to me during that journey.
The key to knowledge is not to rely on people to teach you it.