You may have options if the contingency periods are still in tact. The appraisal process looks at homes sold within the last 3 months within 1 mile of the subject property and then makes appropriate adjustments for size, age, upgrades, etc..
If you would like to get a second opinion, I would recommend Walker Appraisals. http://www.WalkerAppraisalServices.com Ask for Dave or Christine.
The bottom line, your real estate agent and appraiser may be correct on the value but the value ultimately comes down to you. If you truly feel the value is not there for the 50K and contingencies are released, you may very well be risking your deposit to back out. I would work with the agent you have to make this a win-win for all.
If you have not removed your contingencies, the property condition contingency and the loan contingency you are in good shape to renegotiate assuming you have legitimate argument for your case.
Also you should have scheduled to have inspecitons on the property through a seperate inspector. If you haven't you have a right to. You can also ask for a copy of the appraisal and you can contest it yourself if you are aware of the compareables that were used.
The reality is there is quite a bit of liability in dual agency, where the agent represents both sides. If you believe your agent has not represented you fairly then you may have a recourse. I would just approach your agent and communicate that you do not want to buy the property unless the price is dropped, and cite why you feel it is over priced. If he is not willing to work with you, contact his broker and voice your concerns.
Good luck to you.
Always, always,always have a buyers agent represent you, a good buyers agent always runs the comps on ANY property you are going to write on (OBJECTIVELY) and will protect YOUR interests not the sellers. I hope you have not signed off the contingences yet,( your loan and inspections) if you haven't that could be your out. If your contigency period is not up immediately ask for an extension so you can work this out. You could order your own appraisal but that will cost you extra and it still might appraise at that price. I hope you are using your own lender who is working for you. You can always get out of the contract you just might lose your deposit in doing so.
If you feel you have been mis-represented I suggest you consult a Real Estate Attorney to see what your options are in getting out of the contract, but again thats going to cost you extra. I am sorry you are in this situation.
Please let me know if you have any other questions.