Your budget is good enough for a Dual Core server with 1 or 2 GB RAM. The amount of RAM you are looking for would certainly demand more budget. The budget would also slightly increase if you consider a Windows OS and a control panel. Also, there are hosting companies who charge for server management and some offer free server management.
So you need to pay an extra monthly fee to be able to get more services and erm, pay an extra monthly fee for them.
Yeah, doesn't make much sense to me either. I know it's all about making money, but there's no need to unnecessarily confuse potential customers. Just take the £15, divide it by how many "professional" extra packages you have, then add that amount to the cost of each package and scrap the £15 cost.