The big key on the '70 'verts is what they came with from the factory. If its a ho-hum car from the factory, the AOD/302 is nice, but how well done the rest of the car is will set the price. Range is probably $10k to $25k for a base 6 or 302 car cleaned up. Start adding as options like a deluxe interior or AC are there, 351-2V's do a little better, 351-4V's better still, 428's go for big dollars.
As always-it's worth what someone is willing to pay for it in relation to supply and demand. Is there a big supply of 70 verts? no.Are there a lot of people dying for a 70 vert? No. How much are you willing to pull out of your pocket? Is the seller willing to sit on it or is he anxious to sell? If he wants to sell, make an offer and walk away. Good luck.
Deluxe interior is nice if it has it - that's what I have and there were only 1,600 or so made. '70 has a lot of little niceties the earlier years don't always have - collapsible steering column, bigger spindles, foot-operated parking brake, dual m/c. I think it's a great year, but I'm biased.
I'd agree with chpierce's figures - that's what I usually see when I'm looking at them. They have the potential to rise in value because they are in a comfortable range for lots of people to buy but there aren't a ton out there. It's an opportunity to buy something different that has some technical advantages "under the hood" over the earlier years.
I just got a 70 vert a month ago. Recent resto to driver cond, some minor paint chips, good power top, orig.int in very gd.cond., orig H code/3spd-converted to 351C-4v/4spd toploader. I paid 11.5 on ebay and was very satisfied. If you like the model/style, they are very good values now. For a 302AOD, unless exceptional resto/cond.,I've seen a dozen over past month go for 11-16,000. A gold concours MCA went for about 22-24,000,a short while ago. The closer it is to plain vanilla 302, the closer to 12-14 you should be...unless you really like it. After all, if you like it and its a "keeper", whats a $1000 or 2 over 5 years or longer.