I will take for example Poker Stars, the start value for me it's at least 100 dollars.
Because the smaller buy-in on PO are 0,27 cents , 0,55 cents.
So you will have enough tournaments to get some profit.
There are several ways to look at this, when starting out you can put your $ on a site and just play, if you learn from your mistakes well then you may make a profit. However if you are serious, to make poker a job, then it might pay in the long run to pay someone like Doug Polk to join his training program, where they teach you the latest techniques and how to play profitably. They also can examine each hand played and show you what to look for etc.
I have nothing to do with Doug except I do watch his videos and I see he is a very good player. His teaching site is something like upswing poker I think.
I'm Old, my eyes are dim, and my hands shake. So when I take your chips I'll hold them close to my eyes, and mumble.
The higher you can afford the better. Say you have only 20$ USD. You CAN start with that but it will take a LOTS of time and effort to build it till 100$, with proper BRM and assuming you are not extremely lucky.
But lets say you come in with 100$ deposit already. It would save your time and effort and results will be much more meaningful. You can attempt as high as 3.30 buy-ins, although lower would be preferable. But thats much higher than you can afford with 20$ deposit!
If you can afford it, a 300$ would be a good start. Thats what I would do. But STICK to proper BRM nevertheless.