You can still do projections. The cost profile will increase in steps as you add more customers and reach the server limitations and need to add more cost - so there is always that 1 customer that reduces your profits because they are the only one on the server until you fill it up with more customers and reach break-even on that server.
Maybe your accountant is trying to look at server costs as if it were a pure variable cost - so he can multiply the number of accounts by plan cost to make an easy projection. Personally I would avoid that type of analysis - but to calc the cost per plan in that case just take the server cost / total number of plans you plan to put on the server and ignore the fact that different plans may take up different amount of server resources.
Ireland isn't too odd - I've visited Southern Ireland 3 or 4 times and the Guiness is always superb - much better than in the US - I don't think it makes the trip very well