Bowie Rare was released in 1983. I don't know the distribution details, but I do know that I have seen exactly two copies of it available for purchase in my 20 years of purchasing Bowie. One was a cassette copy and one was a vinyl copy. I bought them both, and indeed, I cherish them. It was never released on CD.
That said, I don't believe it's actually "worth" anything, so to speak. Anything is only worth what someone is willing to pay for it, and a cassette copy of Bowie Rare wouldn't command a mint. For one thing, people generally don't shell out for cassette copies of anything, but more importantly, most of the Bowie Rare tracks are available on CD elsewhere. "Ragazzo Solo, Ragazza Sola" is a very nice track to have and it isn't otherwise available.
So, in short, it's a nice recording to have for your own collection, for completeness sake and to have "Ragazzo Solo, Ragazza Sola" (and other tracks if you don't have the Rykos or Christaine F.). But it's a recording you should own for your own pleasure; do not consider it an investment.
It's better to share beer