It depends on the configuration you're interested in, but there are some _relatively_ inexpensive ones from Monoprice [0]. I've tested this one with dual 4k monitors @ 60Hz, and USB 3. It works reliably, which is the most important aspect for me. No laptop charging and fixed TB cable however.
Note that this working will depend heavily on what usb-3 peripherals you're plugging in, and the exact resolution of your 4k monitors.
a display at 3840x2160@60hz will eat up about 19.8gbps. Two of them together will use 39.6gpbs.
Thunderbolt 3 only supports 40gbps. So if you have peripherals that need anything more than basic mouse/keyboard (in my case, a decent webcam and a usb to ethernet adapter) this setup will fail.
I'm no video expert, so correct me if I'm wrong, but I thought you can do 3840x2160@60Hz video with 24bit/pixel with under 14.4Gbps, which is the HDMI 2.0 limit [0].
According to this [1], that signal would require 12.54Gbps, so with a link budget of 34.56Gbps - 2*12.65Gbps, you'd have about 9.48Gbps left over for USB and ethernet (which is on the PCI bus for this particular hub). I suppose these numbers include coding overhead for the physical link, but I can't say for sure. If you wanted more than 60Hz or more than 24bit/pixel, you'd definitely run out of bandwidth with two monitors and USB/ethernet.
[0] https://www.monoprice.com/product?p_id=31262