Cable budget in a rig

As much as needed until you reach the point when you no longer hear any meaningful difference going higher.
And when you buy used you can sometimes get lucky and have $2.5k cables for $600.
It also depends on the complexity of your system. If you have one source component and integrated amp - that's one set-up, and if you have three source components, separate transport and dac, separate phono stage, preamp and power monoblocks - that's something different.
I could live with one of my $500 interconnects on $1200 phono stage but what do you know - I have $1500 cables on it and the $500 one is in the closet for rare cd player use, which is also in the closet. My speaker cables cost new as much as the speakers, and power cord for the integrated amp more than the amp. All bought used except two pairs of interconnects. Now all my active components sound as good as they can, I believe. Soon it will be time to build a completely new system.
However, if you want the simplest answer to your question, I would say 20% new, generally speaking. Tonearm cable is very important if you think about playing records or play them already.
Each cable is a component and all of them much more so.

Thanks guys, my system will be in the 100K price range for preamp, monoblocks, DAC and music server all separates.Therefore i should look at 10-15K for cables ?

This is one seriously expensive system. Cost will also depend on the length of cables and power cords, and also on your hearing. In my view, really good interconnects and power cords start at about $1500 and speaker cables at about $2000, digital cable I am not sure, probably $1000 or so. But you should listen if you can, there is no reason to pay more than needed. One brand approach is simpler but not always the best. Some say that if one runs balanced XLR interconnects, especially if the equipment is fully balanced, the cables can be of lesser quality and lower price. Can be in some cases but I don't take it as an axiom. Sometimes $500 cables are overpriced and $1500 are not, and sometimes both are about right, considering overall crazy prices of good stuff.

Yes this will be my end game system for the next 20+ years.

In one system I have, the cables are roughly 50% of the equipment cost to me, not counting any power conditioning.  This is a lower cost office system, and this is where I landed as providing the best overall sound for dollars invested.

In my main system at home, my investment in cabling is currently about 20%, but I would be willing to go up from there if it yielded significant results.

I would say for a system costing what you suggest, you could build a very reasonable cable solution for 15% of the system value, particularly if you are buying some pieces used.  Understand, with the resolution capable in a system at that price point, you may continue to realize significant added value from rather exotic (and expensive) cable designs, and find the comment that cables are just another critical component in your overall design hold particularly true.

Choosing the right cables can be both exhausting and rewarding.