There are lots of variables but this is how I would proceed:
1) Determine the location of system and decide on monitors or floor standing speakers. I would allocate 50% to the speakers.
2) 40% would be allocated to the source, amp/preamp or integrated. This doesn't necessarily include both an analog and digital source. I like to have the amp and preamp from the same manufacture.
3) 10% for cables
If you can buy used or demo that is the best way to go, then you will have money for the room treatments that will be required in step one.