The current Builder's price is the market price since you can buy it with that price. The price you have paid may not be, since it was the price 2 years ago.
MPAC is just playing a game. The would reject your first request automatically. Next step you go to the tribunal, you pay a small fee for that. MPAC is counting on many people would be unwilling to pay that money and spend time so they give up. At many cases people do give up since the different is not that big. Once you show your will to fight and your case is strong, MPAC usually would settle with you before court day( in my case one week before). The small fee would be returned.
My suggestion is if you have a good case, fight to the end. Don't give up even there is only 100 dollars difference, because the price you get today is the base price for the future.
3) spend $200 to file to file to go infront of a Judge (I forgot the name of the form you need to file) * This is very important, with the number of people dispute their property tax, MPAC will not bother look at your file without you spend the money to this step.
4) Gather your information of why you think you should have a reduction for your property Tax
5) Likely you will get a call from MPAC before the hearing date to seatle the dispute, I have reduced my TAX by 50% doing this, then they really made a mistake on my property from the start.