The sun gear is very cheap ($30 for upgraded version), however, it requires disassembly of the transmission and most places that go into it, will want to replace all the clutches and seals, especially with that many miles. So you are essentially going to pay for a full rebuild despite the low cost of the actual failed part.
As far as GM "new" units, they don't exist. the ONLY way to buy a new transmission is to buy a new truck. ALL GM replacement transmissions are REMAN, even for the dealers & warranty repairs.
GM reman transmissions are pretty affordable. The last 4L60E I replaced was about $1800, which is what most places charge to rebuild. However, with the GM unit, it comes with a 3 year, 100,000 mile warranty, which no other place I have seen offers, and is good at ANY GM dealer nationwide.
So you need to compare what an overhaul from a local shop will charge (unless you want to tear it down yourself) and compare what you can get a GM one put in for.
My personal opinion, if you plan on keeping the truck, is get a GM reman for it.