If you are singing a chorus part, the files for your voice part (for any given opera) cost £10.99. If you have a principal part, then (for the same price - just £10.99) you will receive the files for your part, together with the files for the chorus part which you select.
So, for example, suppose you have the rôle of Zorah in Ruddigore. For £10.99, you'll get the files which have Zorah's part. Let's say you sing First Soprano in the chorus (when you're not singing Zorah's bits). You'll also get the First Soprano files. All for £10.99.
The complete files for any given opera (all chorus and all principal parts, in all versions) are charged at £142.99 per opera, with the exception of Trial by Jury, which is charged at £104.49, and Cox and Box, which is charged at £16.49.