To play as well as you can you certainly need to buy gear, no question there. Whether you have to use your own money or can earn credits for free depends on how much you value your time, where you live and how much disposable income you have.
I know of some who would rather dip into their pockets than do surveys, watch videos and so on. These are the ones who perhaps own their own business and to them time is money, more per hour than they could ever make doing surveys. For them it's cheaper to use their own money.
Earning free credits is also way easier in the US and, to a degree, the UK. I know guys who make 1,000 credits on a decent day, I'm lucky to make 100 and that's only if a survey works. I get the 7 views (which are a joke really) and usually 3 or 4 other videos per day, about 10 credits. It therefore takes me 3 weeks to earn enough for a sleeve of my usual balls. That's why I buy credits now and then and am lucky enough to win prizes in CC tourneys.
And of course, someone with a big bank balance is going to think nothing of using real money to get the best gear.
If you get good enough then the game becomes free anyway, there are thousands of credits up for grabs in RG's every day if your game is up to it.
I do find some of the recent pricing borderline offensive though. A sleeve of balls for 750 credits? A set of Rapture irons didn't cost much more than 2 sleeves of those. Pretty crazy in my opinion.
Where was I going with this anyway? No idea.
I do think WGT is good value for money considering the many, many hours of play an outlay of $100 or so will buy you. If you can earn a few free credits to keep you in balls then so much the better. You just have to set yourself a limit of how much and how often you're prepared to use real money to buy credits. It's all too easy to get carried away.