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If you built a 15X16 shed (less waste material) with one door and one window and depending on how you want it clad, I'd guess the material at anywhere from $600-$1000. Building it shouldn't take that long, but if time is an issue this out of the question. Two guys can frame and clad it in a day and a simple roof about half a day. Plenty of video's out there for the DIY's. If you have no tools and you have never done this kind of work before and you really would like to keep your fingers attached to your hand, then I wouldn't entertain this project.
When I was looking for a shed to stall my sidecar rig, I came up on this site.https://www.dancovershop.com/befr/nl/product/garage-proshed-38x48m.aspx?gclid=EAIaIQobChMIxtL-qOi61gIV18myCh2XfwjeEAQYAyABEgJUFfD_BwESteel, secure and a little over 185 square feetI don't suppose you will be buying in Belgium ;-) but I assume you can obtain similar in the Us?(btw, you can switch country and language at the left above)If you're not afraid of theft, you can also consider a carport and chain you bike(s) to the concrete floor?