How much budget for bass?

Answering your question as a % is difficult because it depends on the price of the main spkrs.

FOr best results you'll use two subs for better reproduction (think of subs as "woofers", rather than SUBwoofers)...

It is indeed said that, "bass is the foundation of Western music". Very few systems reproduce bass and we're generally accustomed to missing out on quality low-frequency reproduction... more's the pity.

So2 subs Wow! How do you connect 2 subs?

I prefer primary speakers which do not require bass augmentation (I'm talking non-A/V systems here), and there are many to choose from. G-o-o-d bass? Too many have been weaned on exaggerated bass such as that dreadful sound in cineplexes (yes, there are movie theater venues with good acoustics, but they are few and far between). I always tell fellow audio shoppers to first attend folk, Jazz, classical performances in various intimate indoor venues. Most over twenty-something know what outdoor concert sound systems sound like, but most will not be replicating that effect in their homes. For those with room correction software/equipment who disagree with me, we all have our opinions.

The best money you will ever spend is on room treatments. If bass is a concern, invest in some quality bass traps. The best speakers in the world can sound just okay in a bad room.

Only one observation here:

High qulity bass in most rooms requires a lot of effort, because this is the range where the room is putting up a real fight. If your speakers are within 5ft of a reflective surface (wall), expect serious issues below app. 150hz.

Bass traps IME can be pretty effective down to app 80hz or so (best case), but below that, you're into high tech (DRC) type solutions and/or multiple subs (2 is better than 1, 3 is better than 2, 4 is better than 3, etc.). This is an expensive proposition, relative to midrange reproduction where the room does not "interfere" nearly as much.

Whether fighting this fight is worth it is a personal decision that rests on individual taste and priorities, preference in program material (if you listen exclusively to solo flute, it's probably not worthwhile to spend a fortune fixing the bass), room size, etc.

In the end, no-one can decide this for you.

Marty