I went to a silent auction at our local High school where I paid $90.00 Cash for an item i bid on ? can I claim the whole 90.00 since it was cash?
No. As a charity donation, you can only claim the amount that is more than the fair market value of the item you purchased. If the item is worth $80, then only the $10 "overpayment" is a charity donation. If the amount is worth $100, then nothing is a deductible donation because you paid less than fair market value.
Furthermore, the organization is required to give you a receipt or letter of acknowledgement when you pay them $75 or more, that indicates the value of the benefit you received, and any tax deductible value. If you don't have that, you can't claim a deduction.
No, you can't deduct the entire $90. It doesn't matter what the form of payment was. You can only deduct $90 minus the fair market value of the item that you received. If the fair market value of the item is $90 or more you have no deduction. To claim the deduction you need an acknowledgment from the organization stating the amount that you paid and the value of the item.