The ROOL team recently announced that they're no longer registered for VAT, which means the Norcroft C compiler and associated tools are a bit cheaper at GBP50 (plus P&P). ROOL took over the distribution of the suite from Castle last year, who took them over in turn from Acorn.
The Norcroft compiler has long been the default option for C programmers on RISC OS. It's only relatively recently that the GNU Compiler Collection (GCC) has provided a genuine alternative. The latter has many advantages, including development drawn from a wider pool of developers and the ability to compile RISC OS programs on fast PC-based hardware. However, at the time of writing, the RISC OS sources are only compilable using Norcroft. It's possible this may change in the future, but for the time being, if you fancy getting involved in the porting efforts, obtaining a copy of the necessary tools just got a bit easier.
The ROOL announcement is here. The announcement was publicised on RISC OS Info a couple of days ago.