When will HD 5870's prices drop?

When nVidia cuts their prices or when they release release their new video cards.

How long a video card lasts you will depend on how much performance you expect down the road. If you can live with medium quality settings in 2011/2012, then the HD 5870/5850 should be fine.

Is about 10% - 15% increase in performance with the extra $100 to you?

I would say no,
for the price I would really get a 5850
the difference isn't that big to spend another 100 dollars on it
but then again, if you have the money, and willing to spend it,
you should, if you want to keep the rig that long

Okay, I can live with medium quality, even low. Because I'm typing this with my computer which is very old, Intgreated graphics controller, 82865G. And Intel celeron 478 socket, 2.8GHZ (now 3.0).

...lol
he'd better buy the rest of the system as well...

I would say only about $20 - $40 drop or so before Christmas, their selling like hot cakes right now and sold out everywhere. My guess it will be a few months into next year before you see it drop down to $300 or less, buy then the 5890 model will be coming I'm sure just like the 4870 and 4890 did last year.

so is the 5890 going to be $500? (judging from the other prices on the other ATi video cards)

5890 will be made if Fermi threatens ATI really bad, and i highly doubt it is worth buying since waiting for a HD6xxx would be better.

likely only if Nvidia will launch the femi before the end of the year as promised. if they dont i would not borther much about it and either buy a cheaper card such as the 4870 or then wait until 2nd q 2010 before upgrading. 4870 should not drop that much in price anymore. And you will be able to sell it off for lets say €60 if you bought it for €110. radeon 5850 on the otherhand is well in demand now but by that time you can expact 5870 to be priced atleast around €200 if not €130-150. there will be a 5890 too someday. Nvidia just is slow at releasing there part since it is mainly aimed at the €5000 a piece costing super computer market as the tesla product. a market which is much more profitable. However the gaming market is still a good market for them to run next to it. AMD looses marketshare on the FireGL market once fermi comes out i can guarrentee you that they will. cost is not imporment there but processing power is. Fermi might not end up the fastest card if 5970 comes out. But that is only speculation as most of my comment. But you can expact the prices to drop within a few months. AMD might be increasing the prices now but they will soon have to drop them once Nvidia has a product to compete with.