For a shipping container in good shape, expect to pay between $0 and $4,000. Seriously: if you're patient, sometimes you can find difficult-to-move containers for free.
Find somebody local selling them on craigslist; conex is an industrial term that some sellers use in their listings but most sellers don't know to search on. I bought my first container (of 4) for $1,200 delivered in Austin, TX; I'm convinced I got this deal because the ad showed up in searches for 'conex' but not 'shipping container'.
If you're shipping the container overseas, just buy it from your shipping company: it has to be certified, or cargo ships won't ship it. I used Finn Container
, but I would NOT recommend them: little to no support with customs.
For more detail, see my Instructable on getting a shipping container