Hard pill to swollow, but if a person had a paid for house, they still have to pay property taxes. The worth of your house is what the taxes are based on.
Sounds like you got a fabulous deal and the $105 per month in taxes (depending where it is) is a small price to pay in saving $100k! Congrats!
have the realtor that you used to buy the house pull the comps for you, and then you'll have to get in touch with the assessor's office and file for a formal review of your property. What I have seen personally is that in most cases you can get them lowered a bit but almost never down to the purchase price, best case scenario is to split the difference, another way would be to call an appraiser and have them come out and give their formal opinion of the place in writing, then of course you may have had an appraisal done for your mortgage and you can use that.
best of Luck