We might have a server with 1200GB bandwidth and 40GB hard drive which costs us $69. WIthout much overheard we dont have to charge much because our resources are cheap.
On the other hand you might have a fully managed server for $399 a month with only 250GB transfer... there for you might have to charge $14.95 on something we would only charge $7.95 (vice versa).
Take your total costs (server, software, licensing, staff, phone #, etc). Then give yourself a specific profit margin (I do 30-50% depending on the services). Now that you have a rough figure of what you want/need to make... take that number and divide it by the amount of plans that can comfortably fit on that server and you will get a ballpark price of what each client needs to pay for you to reach your goals.
Of course everyone does things different and this is a very basic way of planning out your pricing, but it works.
I feel that realistic is a relative term. What I feel is realistic is complete 180 degrees of what you may consider it to be. I would say a plan is as realistic or unrealistic as the needs of the person looking to buy it compared to that of the wants of the person selling it. The actual setup numbers of the plans are arbritrary if the two do not meet and form a tangable hosting agreement.