lol, that is a trick question, what is good condition to me may not be the same for you, what options, which engine, there were 3 V8's with a 4 barrell....
your question is too vauge, there is too much that goes into the price of the classics, like if it is an auto, power steering, standerd interrior, rarity....the list goes on and on
plus, asking for a price without actually inspecting a car is not a good thing since it might have worn out suspension, worn out clutch linkage, massive ammounts of bondo, rusted floors....
Price can vary a lot depending on where you are. Use www.nadaguides.com for classic cars. You plug in your zip code and the values take where you're at into consideration. NADA gives three prices, low, mid, high. These relate to #3, #2, #1 MCA condition.
twister has a major point... some of the old mustangs are worth way more than others.... best advice i can give you would to be to find another simlar mustang and see what it prices for... you might try ebay or for sale mags
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66' Ford Mustang 289 restored 4 speed top loader. intake man. carb. cam. 9' pozi. disc brakes. new susp.
67 fastback price.. 
