Normally if you have a valid Social security number and atleast $3000 in income in 2007 than you would receive atleast $300 in stimulus check payment. The calculation of the stimulus check also considers the taxes you have paid in 2007. Since your gross income was only $15,000 you must have not paid any taxes in 2007. Hence, the rebate check given to you was $300 for you and $300 for your spouse. If you had taxable income and paid atleast $1200 in taxes than you could have received $1200 in rebate checks. The calculation of the rebate check is a bit complex and confusing. You can use the calculator available at the IRS website at the link below to determine if the amount you received is correct-
Let me know if you have any question.
Please note: This advice is provided with the understanding that all the relevant facts have been provided by you. Any change in facts might affect the advice given and hence may not be relied on in such cases. Nothing contained in this reply was intended or written to be used, can be used by any taxpayer, or may be relied upon or used by any taxpayer for the purposes of avoiding penalties that may be imposed on the taxpayer under the Internal Revenue Code of 1986, as amended.