SSDI amount you receive is based on your age and work experience. You get a 1099 at the end of the year showing your total SSDI amount and you take your total income (1099 + any other income) divide it in half* I think and if it's less than $25K than you don't have to pay any taxes. I get $1600/mth on SSDI over last 10 years and my total income at the end of the year has always been non-taxable. (I was lucky that I had several years of a half-way decent salary before applying for SSDI which is why I get the SSDI amount that I do.)

*There's a worksheet in 1040 tax form where it shows you how to figure out if you owe tax on your SSDI and I'm pretty sure you divide your total income by 2 and if it's less than $25K you owe no Federal taxes. You could download a IRS 1040 instruction form and find the correct worksheet and you can see how it is calculated.