What a shed budget usually misses
The foundation is where shed budgets most often go wrong, because it is the part you cannot see in the finished photo. A gravel pad needs the ground levelled, a border to hold the stone, and enough depth to drain; a concrete slab adds forms, reinforcement and a delivery that has a minimum charge whether you use it or not; timber skids are the cheapest option but only work on ground that is already flat and well drained. Before you price any of them, check your local rules: many places require a permit above a certain floor area or wall height, and most have setback rules controlling how close to the property line and to your neighbour's fence the shed can sit. Moving a shed after it is built is far more expensive than getting the position right first.
After that, the extras add up quietly. Site access matters, since a truck that cannot reach the spot means everything is carried by hand. Levelling a sloped yard can cost more than the foundation itself. Running power out to the shed is electrical work with its own permit and inspection in most jurisdictions, and it should never be assumed to be included in a builder's price. Then there is the finishing that turns a shell into something usable: a ramp, shelving, a proper lock, guttering, and paint that has to be redone every few years. If you are pricing a kit against a build, compare what each includes down to the fasteners, and hold a contingency of roughly 10 to 15 percent for the ground being less level than it looks.