Short answer, put down $900 on the form.
Long answer - estimate how much fabric for backing and top, multiply by cost of fabric; example 8 yards times $10 = $80. Same thing with thread and batting. Estimate how much time you spent cutting, sewing, quilting and multiply by what hourly rate you think you're worth. Add all figures together.
i made a quilt for my MIL and my husband told her to insure it for $7000; he did the math and figured out that's how much it would cost to replace. (It had over 9000 pieces in it.)