Is it common for kronor to actually be translated?
Im not sure what other people do but as a Brit, I, and my friends, tend to translate it into crowns when talking in English. This may be influenced by the fact that we used to use crowns as a currency in England so it's not so strange.
Why not "av kronor"
How do you know when to use av and when not to?
Could one also say "Leksakerna är hundratals kronor värda"?
Nope, doesn't really work in Swedish.
What about 'hundratals SEK'?
Not really - that'd be like saying "USD" instead of "dollars".