If you're thinking of building a timber shed from scratch...
I've worked out the costs for mine, It's 12x6.6x3.0m, on timber posts in concrete stumps. Coupled roof, exposed rafters and wall framing. We're on highly reactive black soil. I work at a sawmill so I get wholesale price on timber
Plans: $990
Soil test and Stump design: $715
Owner Builder Licence: $275
Council Approval: $460
Post hole boring: $500
Post hole concrete: $3620
Timber frame: $5500
Weatherboards: $4600
T&G Floor: $3500
Colourbond Roof: $2000
Windows: ?
Electrical: ?
Not including the windows and electrical (which I haven't got a price on) the total so far is $20360.
I doubt a tin shed on a slab would be much cheaper, consider how much concrete and reo would be needed