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If you can't pay fees required by immigration forms - you may apply for a Fee Waiver - see more details - http://www.uscis.gov/feewaiver
Persons financially unable to pay the base filing fee or required biometric services fee may submit a feewaiver request as described in 8 CFR, section 103.7(c).
and Fee Waiver Guidance - http://www.uscis.gov/files/pressrelease/FeeWaiver072007.pdf
let me know if that helps.