If you're trying to go fast af, you could get it under r24 hours but that would cost around 300-500m.
If you're doing the slowest method possible, it could cost only 12m. Depends what you want to do.
54-96 is a big range. There are so many possibilities.
To get 84, it's about 62m if you go along with this guide. I'm not entirely aware of any rather cheap methods after around 85, I spent quite a bit on 99 Herblore myself. I spent about 215m on 88-99 I think, so it'd probably at least cost you 150m doing extremes to 96. If you're a patient person, you could grow your own herbs and do whatever method you want.