I bought my copy when the 3.1 was released, even I have to pay 300bucks for an update to zbrush5, it is ok to me, I love pixologic and what they doing for the industry.
I wouldn't worry about it considering Zbrush 5 is probably over a year or more away. Pixologic has long development cycles. At any rate, they'll probably offer upgrade pricing if they do charge for it, so you won't have to pay full price again.
If Pixologic start charging for upgrades I don't think it will cost more than it's nearest competitor Mudbox (about $500 perpetual, $10/month rental). I think they'll still offer a free version and maybe have a bundle (two-way Modo bridge?) to sell.
Even if you have to pay for an upgrade, it almost certainly wouldn't be anything near full price. Speculating based on watching how much the 3D community talks about Zbrush compared to it's competitors, and how Pixologic has stayed afloat with new sales alone, their market penetration must be like 90%. If they charged $20…
Upgrades have been free since version 1 - released over 10 years ago. At some point pixologic will charge for an upgrade. I'd bet money this time the next full version will be a paid upgrade. When that will be I couldn't say. This is the risk you run with buying any software though, zbrush being the only exception so far.…