Server AMD 6100 vs Intel 5500/5600

I run multithreaded numerical calculations and am considering the upgrade path with AMD vs Intel server cpu families. I am generally not looking to pay for premium processors ($1000 and up), but for the value ones $600 or less.

I am primarily motivated by !/$. I note that there is a premium associated with intel, however intel does have faster cores. AMD potentially makes up for much slower cores by having many more. Some of my numerical work is memory intensive, so not sure whether will effectively scale to 24 cores on the AMD if the memory / core interconnects are not up to the task.

Long story short, should we expect to see the higher end X5500s or X5600s scale down in price as new processors become available? I recently built a dual E5520 system, but want more performance. Not willing to pay $1600/cpu at this point.

I also have an old dual X5150 (mac pro) based system and noted that the cpu upgrade (the X5355s), which are not even great performers cost more than the E5520s I purchased ($450 for X5355 vs $380 for E5520). The specfprate difference is something like 40 vs 80 per cpu.

Is this an anomaly or do Xeon processors hold their initial retail value even as new processors become available?

Sorry to be long winded. Just trying to understand which family is likely to provide !/$.