Your deadline for protest is June 2, 2008, or 30 days after you received your notice of preliminary assessed value, whichever is later. Nearly 99 times out of 100, the appraisal district has to use the purchase price for the home if it occurred in the last year since that establishes market value. It is extremely rare for the appraisal district to be able to defend that a market transaction was "a bargain". Likely, in these cases the tax payer is simply inexperienced and the appraisal district simply took advantage of them. If you are still able to protest, you should be given the option to obtain all of the comp data the county appraiser used in establishing the value of your home. You will then typically be given the option of an informal hearing, and a follow up formal hearing. Take your HUD-1 closing document to the informal hearing and ask the appraiser to defend why that does not establish your value.
Keep us posted by replying with answers on the status of your effort.