That is in an ideal world, you still dont get away from the rfl on the zed, mine and jims are £425ish, the multicar insurance that will add an additional cost,plus you will have a further mot cost and rfl cost, you will rag the nuts of the zed when you do drive it at the weekend so your fuel will cost just as much for a weekend rag about plus the fuel you used in the shed all week, as opposed to just using the zed with a light right foot all week.
So... in summary. Its not worth having a shed if you still running a zed at the weekend, tbh it would probably cost you more in the long run.
Now however.....if the rfl was on top of the price of fuel and not a stand alone cost, it would make sense having a shed. I think anyway.