Quite honestly, I wouldn't worry too much about cost breakdown on source and cables. Quite a few responses above centered on what the OP didn't ask, and didn't address what was asked.
To save on money and heartache, I would suggest that you borrow midrange audioquest, cardas, or other nice cables and try them in your home. Dealers have demo cables lying around that they are trying to get rid of. Ask to borrow some for a short period and try them.
Get a baseline on what is acceptable in your system and go from there. The upgrade wheel will always be turning, so, borrow first.
as far as sources are concerned. What exactly are you interested in?
CD player/transport?
Streamer?
Turntable?
I read a few of the earlier responses and got frustrated that many didn't address your original concern and didn't read the others, so apologies if I missed responses to these concerns.
Many good dealers will let you take equipment home (with a credit card deposit on file) so you can "demo" equipment in your house. Find a dealer that will do that.
Determine what "source" you are concerned with and borrow one. A mid level Aurender, etc. and try it at home.
When you get to the point of "I'm happy with this sound", good. Live with it for awhile and then borrow equipment slightly better quality and try it in your home system. See (hear) if you can determine a significant enough difference to "justify" purchasing.
One thing to be aware of. Match levels first before listening to new (different) equipment in your system. Simple gain level differences can be mistakenly perceived as better or worse.
I always listen at a certain level with the original equipment. Then I Place a test CD or album on a play a 1 khz tone, use a inexpensive sound level meter and gauge the level at the listening position. Then I swap in the new piece of equipment (one piece at a time), play the same test tone and adjust the level to match the previous level before seriously listening.
Determining a particular costs percentages is in my opinion not accurate. Find pieces you may be interested in (expensive and mid priced and inexpensive) swap it into your system (one piece at a time), level match and listen. listen to music you are well familiar with that is revealing. Then ask yourself, is it better, worse or the same as what I have and does it justify me coming out of pocket for a change.
The sound quality actually determines the price point, not the other way around.
enjoy