The u-blox F9P definitely works well, given a good antenna, good sky
view, and good reference data. There is also a Septentrio product
family "Mosaic" but I have not used it. I am not aware of other
sub-$1000 GNSS receiver chipsets that do RTK. There are of course
things like the Trimble R12, and while they don't seem to post prices I
am sure they are well over $1000 :-( and I have no reason to think they
work with gpsd, or that anybody using them would be using other than an
expensive proprietary-software data collector.
https://www.sparkfun.com/products/18442
https://www.ardusimple.com/rtk-receivers/
https://geospatial.trimble.com/products-and-solutions/trimble-r12i