The cost of Plan B (or any other health insurance product) would be the same for all group schemes. The yearly cost would vary depending on price increases and the like and there might be a variance on any given date because of renewal dates. But at the end of the day, if Plan B is €900 then at some stage all members will have paid that.
You should be able to get the costs quite easily from VHI, either by telephone or email.
As for the renewal dates and the Revenue's year running at different dates, Revenue should be au fait with this. Just tell them what the gross premium was at each renewal.