I don't know whether you saw this: http://typewriterdatabase.com/Royal.P.72.bmys

Indeed, a Royal model P, and made in 1930 based on the serial number. There are a lot of these still out there -- well made machines but not especially valuable -- $50 US give or take. Condition is important, both cosmetic and working condition, and whether it has a case or not. I suggest you try using it! Get a new ribbon (it takes a standard ribbon and spool). Here is a link to the manual: http://site.xavier.edu/polt/typewriters/Royalportables1930.pdf

That's hard to say -- it's not as if Royals were made in small quantities. The model P such as you have came in a variety of colors, which were and are pretty handsome. In the 1950s Royal tried to jazz up sales by producing the Quiet De Luxe (and maybe other models; I'm not sure) in a variety of bright colors, and those seem to be sought after for their looks. But again, I don't think these are rare in absolute terms. (Oh, if you happen to find a gold-plated Quiet De Luxe, those are definitely pretty rare. Ian Fleming had one.)

One thing to check on your P -- see what condition the platen is in, and also the feed roller. I mention this because I recently got one of these myself and both need work -- the feed roller has a flat spot, which hinders smooth feeding of the paper.  

Not sure where you are located, but if you're in the USA and there is a Staples near you, they do carry standard ribbons and spools (Porelon brand, #11579; the box is labeled as being a refill for a printer, but it will fit perfectly). Note that it's not red and black, just black.

Ribbon in the UK? In the UK, try Kleen Strike, Brian Rothwell, or Office Buddy.HTH