I too have a recently installed Thumb on my Kubota
M4700. Cost me approx $700 total, including running wet lines from rear remotes and having my neighbor do the metal work. I figure that's anywhere from $500 - $1500 less expensive than a stand alone
grapple, depending on quality, etc. FWIW, the Thumb is built like a shickbrithouse.
The Thumb actually offers some advantages over a stand along
grapple. For one, you can don't have to switch implements whenever you want to switch between bucket and
grapple. For another, the spread between the fully open thumb (I have the 36" model) and the bucket lip is 5-feet, a wider opening than most stand alone grapples. This has been huge for me, since I move round bales regularly. I'm not certain it'd be so simple with a stand alone
grapple.
I'm sure a dedicated brush
grapple would be better for moving brush. But I can still move huge piles of brush, logs, round bales, and just about anything I can think of with the Thumb for half (or less) of the price and no changing implements.