I'd expect around about £5 to be a reasonable price for a 2-3" Red Devil most pet shops / LFS sell.
However, most aren't pure Red Devils they are a mix between Red Devils and Midas Cichlids. A PURE Red Devil has very thick lips, where as most of the crosses between Red Devils and Midas will not possess these large lips.
Take these fish from Jeff Rapps for example:
Red Devil
Very thick lips.
Now a Midas Cichlid
A rounded, less pointed head and lack of the thick lips.
A Red Devil x Midas will have a more pointed head but lack the lips. Its very difficult to get a hold of a pure Red Devil or Midas unless you have a reputable source.
On a side note - Oscars, although they may be able to cope with a Red Devil once bigger, I don't think will do too well in the juvenile stages, especially as the Red Devil's aggression grows, I think you'll be looking at some pretty beat up Oscars, but hey, I could be wrong.
Good luck!