What do you want to get out of this game? Do you want to be as good as you possibly can be or do you want to purely play for fun regardless of how good you are?
If you merely play it for a bit of fun every now and again then not spending any money is the way to go. If however, like a lot of players, you want to become a Master and be one of the better players, you have to spend some money.
I have spent in the region of $200 on this game in the last 6 months. A lot of that was spent playing Ready Gos at a time when I probably wasn't good enough to regularly win credits. 6 weeks ago I turned the corner and have now got to a standard where I do not foresee having to make any further deposits to WGT to buy balls and enter premium competitions.
Invest in yourself and if you become good enough you wont have to spend any further some time soon.