The downside is that it's downright ugly and it doesn't last very long. Five or eight years is the life expectancy of roll roofing.
Cost: $35-70, based on one or two rolls of 100 sq. ft. each at $35/roll.
2. Shake ShinglesThe other school of thought mentioned before is that your shed may be highly visible, so make its roof as attractive as possible. Consider this: Your shed is visible from your house, you are in your house a lot, and the shed's roofline may be low. Even if you aren't trying to win awards with your shed's roof, at least you don't want it to be unpleasant to look at.
I once had a common potting shed shingled in gorgeous cedar shake, and I enjoyed the style. Since I had to look at this shed numerous times a day, it was well worth the cost. Had the shed not been as visible, I am not certain I would have had shake on my shed roof.
Where to find cedar shake on the retail market? Forget The Home Depot and Lowe's; they really aren't interested in stocking shake. WoodRoof.com, based in British Columbia--where all those forests are located--is an excellent source of western cedar shake shingles. Though you will have to contend with shipping costs, you will not be buying much shake--one or two squares, tops.
3. Asphalt ShinglesAsphalt three-tab shingle is the compromise. Lowe's and Home Depot will happily sell you asphalt three-tab, though limited to basic tans and blacks. For most homeowners, even a shed that is not made of corrugated metal is considered an upgrade. So, these big-box offerings will be just fine. Toss the bundles in your car trunk and save shipping costs. Be careful, though, each bundle of asphalt shingles weighs about 75 pounds and is unwieldy to pick up.