What they cite as the cost per plane is always changing based on their projection of how many planes will be built over the life of the assembly line. That's why the price of the B2 was so high. The R&D was high, but then they only made about 18 planes. The cost of R&D + mfg cost of 18 planes divided by 18 was a couple of billion per plane. If they'd left the assembly line in place and made another couple of hundred planes, the R&D cost would've been spread out over more planes, driving the average cost down.
The irony is that every time the bean counters say, "This plane costs too much; you have to buy fewer," they drive the cost even higher because the R&D is spread out over fewer planes. The same thing is happening with the cost on the F-35. If we decide to build fewer, the price goes higher, making our allies say, "We can't afford this plane now, we have to buy fewer." The dog just cases its own tail faster and faster. Airbus is facing this problem with the A380.
Obviously, the F-15 will lose to an F-22, and a group of F-15s is doomed to a group of F-22s due to the computerized integration of battle strategy. The F-15 will surely not dominate F-35s either. The thing is, we may not sell the F-22 to anybody else (maybe Australia and UK), and the F-35 only to our allies. Any F-15s kept in service would not be facing our own planes. Keeping old F-15s for homeland security is cost effective in that a hijacked commercial airliner will always be easy prey for an F-15. It would be a long time before most enemy countries would have something capable of engaging the F-15. (The Eurofighter will likely not get sold to our enemies. The SU-37 was only a prototype and was cancelled due to too high maintenance on the thrust vector device. Russia does have the Mig 29, and though sold to some of our enemies, third world pilots in a Mig 29 just don't have the skills to take down a USAF pilot in an F-15). The F-15 could still be useful in the wild weasel role, just as have the F-111, and until not too long ago, the F-4. So I do think in the roles of homeland security and possibly in a wild weasel role, the F-15 may still have a role to play for some time into the future.
Here's a great air combat site:
Air-Attack.com - News & Facts on Military Aviation and Space Projects