If you're gigging rock and need to keep afloat between guitar halfstacks and loud drums, I'd say that the bass/amp ratio when you buy new from the shop would be around 1:2.
When you're roughly in the same quality/bang for the buck area, you spend around a thousand bucks on cab and head, plus around 500 on the bass.
That whole equation gets tossed around a bit as soon as second hand items are allowed in.
You can get a decent head and decent cabinets for next to nothing if you're going for solid state amps from the 80s/90s and the cabs that went with them. They are big and heavy, and compared to today's amps, not super loud (but hey, instead of one 410 just bring two and you're golden). If you don't mind the schlep, you can make a dollar stretch a long way here.
Also, with that 1:2 ratio on will be set and ready to go. As soon as one takes the first steps on the path, everyone will experience different things. One might feel underpowered and wish for more cabinet or more watts.
The next might feel that the amp is fine, but crave for a better bass.
There is no right or wrong here. As soon as your equipment meets the minimum demands ( the stuff is reliable, the amp goes loud enough, the bass is playable and intonates right), the upgrade path is up to your financial situation and your personal taste