How much does 99 Herb cost?

Or at least till I can use overloads? I already 99 prayer and 99 magic. I want to get 99 range and then 99 herb. (I'll train my main combat through just slayer)

 

I suggest you get 99 herb before 99 range so you can use overload's or at least extreme range's.
Also, it depends on what lvl your herb is already, but when I got my 99 herb back in the day (2008) 99 herb cost me approximately 200m - 225m from like lvl 50, but I'm pretty sure it's a lot cheaper nowadays.

 

55 - 99 Cost me 374M.
55 - 81 Pray Pots.
81 - 99 Sara Brews.

December 2010.

 

You could probably do it for 20m with prayer pots and them decanting them.

 

http://runescape.wikia.com/wiki/Herblore_training

There you go.

 

How long is a piece of string?

You probably don't know. That's pretty much your question. A string varies in length and you cannot answer that.

The same goes for Herblore. You can profit, if you clean herbs, or you can lose if you make pots. And we don't even know you're level, whether it be 1 or 98.

Use this, it's a calculator for potions. And this is a calculator for cleaning herbs, keep in mind that herbs aren't stable and you could lose money. So I suggest you buy a few grimy herb(s) and sell them when cleaned, to see if you profit.

It isn't hard to do the math yourself, or get an estimate. I hope you don't ask people to do the math for you.