USA same as UK,

Proof of Financial Support and Affidavit of Support forms

During the visa interview, applicants will be required to present evidence to the Consular Officer that they will not become a public charge in the United States. You may present evidence that you are able to financially support yourself or that your U.S. citizen spouse is able to provide support. The Consular Officer may request that a Form I-134, Affidavit of Support, be submitted by the U.S. citizen spouse.
You have to have an income of 125% of the current Federal poverty guideline for your household size,

2014 POVERTY GUIDELINES FOR THE 48 CONTIGUOUS STATES
AND THE DISTRICT OF COLUMBIA Persons in family/household Poverty guideline
For families/households with more than 8 persons, add $4,060 for each additional person.

1 $11,670
2 15,730
3 19,790
http://aspe.hhs.gov/poverty/14poverty.cfm

Looks a lot lower than the £18k6 a Brit needs looks around £13k for a couple