So for 4 DIMMS slots filled it's 2400 default support. For 2 DIMMS slots filled (with single rank DIMMS only) the default is 2666.
Running 2 DIMMS at 2400mhz will NOT decrease performance. Like i said above, faster ram DOES have an effect, and increase in performance in certain situations. Getting 2400mhz DIMMS is just fine. The preference being to get DIMMS up to 3000mhz, but like you said, price/cost is key.
edit: Here's an example. Lets say your playing game A with 2400mhz. You might hit 80fps at whatever settings. If you have 3000mhz DIMMS, you may get 85fps, not so much a difference, and one that you wouldn't be able to see anyway. Do you see what I mean now?