MudBox is $795.00
http://www.autodesk.com/products/mudbox/buy

zBrush is $795.00
http://store.pixologic.com/ZBrush-4R6-Single-User-License/

So far zBrushes upgrades are free.
Not saying I haven't but
I can't remember paying for a zBrush upgrade in 14 years.
who knows about 10 years a go thou.

Looks like MudBox anual fee upgrade $555.00

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The Artist that will fight for decades to conquer their media.
Even if you never know their name ,your know their Art.
Dark Sphere Mage Vengeance