SSDI:
During the trial work period, there are no limits on your earnings. During the 36-month extended period of eligibility, you usually can make no more than $1000 a month or your benefits will stop. But, the work expenses you have as a result of your disability are deducted when we count your earnings to see if they can help you keep more of your benefits. If you have extra work expenses, your earnings could be substantially higher than $1000 before they affect your benefits. This substantial earnings amount usually increases each year.
We deduct work expenses related to your disability from your earnings before we determine if you are still eligible for benefits. These expenses may include the cost of any item or service you need to work, even if the item or service also is useful to you in your daily living. Examples include prescription drugs, transportation to and from work (under certain conditions), a personal attendant or job coach, a wheelchair or any specialized work equipment.
http://www.ssa.gov/pubs/10095.html#part3
SSI:
The amount of your SSI payments is based on how much other income you have. When your other income goes up, your SSI payments usually go down. So when you earn more than the SSI limit, your payments will stop for those months. But, your payments will automatically start again for any month your income drops to less than the SSI limits. Just tell us if your earnings are reduced, or if you stop working.
If your only income besides SSI is the money you make from your job, then we do not count the first $85 of your monthly earnings. We deduct from your SSI payments 50 cents of every dollar you earn after the $85 deduction.
Example: You work and earn $1,000 in a month. You receive no other income besides your earnings and your SSI
$1,000
– $85
$915 divided by 2 = $457.50
We would deduct $457.50 from your SSI payment.
You may be eligible for a “plan to achieve self-support” which allows you to use money and resources for a specific work goal. These funds do not count when we figure out how your current income and resources affect your benefit amount.
http://www.ssa.gov/pubs/10095.html#part7