This is only correct for each item valued over $5,000. For items valued at $5,000 or less you do not need to declare your cost basis for that item. See form 8283, Part I: http://www.irs.gov/pub/irs-pdf/f8283.pdf
Also, as to not exceeding the cost of the storage unit. You actually can go all the way up to the cost of all units purchased during the year, as well as, all expenses incurred to makes those purchases. Technically, the buying of units is considered a business, and being such anything to do with the IRS is based on the gross yearly income and all expenses to create that income.
Have you read this form?
It's for exactly opposite of what your saying.......
For deductions over $500.
With a section to itemize. Where you got, your cost, FMV........
............
Now the other part of your post is where I get confused on finding cost of items.
Yes, your correct. Total cost (unit, gas, dump....) is taken from sales to get AGI. But I'm not sure how to value items left based on costs.....
I've yet to find any direct formula on calculating this.
I mean, think about my prior statement on donation can only be what you paid, not what FMV is.
But now, you go to sell business. They will add up FMV and tax you on the sale....? Really, the price of business should be calculated on what you paid, but it's not. Tax law sucks.