The assessed value of a property, relative to the asking price, is meaningless. The vast majority of taxing entities use something called "mass appraisal", where a computer model is used to determine the value of the properties in a given area of town. It is not unheard of for a taxing entity to actually have a practice of being accurate on their assessments, but it's very, very, very rare. Again, assessed value is meaningless.
That said, what the agent provided you, according to your question, is also meaningless. Go get an agent to represent you and have them put together a market valuation for the property you're interested in. They will pull sold data for similar properties that have sold in the last 6-12 months. Sold data is the most valid determining factor for what a similar property is worth.
Best of luck,
Don't waste your time on this one...they're way too far off and unrealistic to even consider negotiating with them.
Sounds like lots of good advice below. If you like the house, keep negotiating. If you think it's a value issue, then you can negotiate and make sure your contract provides for an appraisal that equals or exceeds your purchase price. If it doesn't, then you can walk away (provided the contract is worded correctly). Keep in mind, however, that you would be out of the price of the appraisal and home inspection (if you get one). Let us know how it turns out.
Oh, and tax assessments are usually done on a very mass scale and frequently incorrect. Case in point, a property I had at 4800 sq. ft., 4 1/2 baths, 5 bdrms was assessed at 3500 sq ft, 3 1/2 baths, 4 bdrms. The tax assessment was over $100,000 less than market value. Means less taxes!
I have to second what Jeffrey has said. The tax assessed value is only valuable tax purposes but when it comes to the selling price of the actual house, it would not act as much more than a guidepost for prices but defiantly not an actual price. As you know, generally, even in this market, houses are selling for more than their TAV (unless they want to sell in a HURRY). So, I also second what Jeffrey said in finding a competent agent. Comparable sales data is the best way to determine how much a house should sell for apart from an appraisal. If at all possible, have your agent pull up "comps" for similar sales within the past 6 months. If he/she can't do that, pull back to a year for comps. Don't use information of houses that have sold for over a year because these don't matter to appraisers. I hope that helped.