Why do you say a Pentium D is way cheaper than an Athlon X2? Are you trying to compare them at the same processor speeds?
As you may have noticed, clock speeds don't really tell you as much about the CPUs as they used to. It is the model number that needs to be looked at, and that still is very confusing. Maybe in the future the CPU makers will give us an easier to understand system, but for now the only way to know is to do the homework.
Here is a really useful interactive chart to compare CPUs. Have a look:
http://www23.tomshardware.com/cpu.html?modelx=33&model1=235&chart=61&model2=201