Thing is, whether is is some clutch pack, the converter, or valve body, chewed up gears, bands, burnt seals, broken accumilator springs, whatever, the transmission has to be rebuilt either way.
It ain't like and engine where you can fix JUST the head gaskets or whatever.
Don't take it to some shop that will "patch" it. Something burnt up or broken, rest assured there is contamination throughout the thing. Fixing JUST one clutch pack or the valve body is not going to be a reliable repair. (tho some shops do this)
Is it safe to drive - It CAN be driven, but it is likely to give out at any time. You will coast to a stop at that point (hopefully not on some highway) and then have to have to towed.
You might want to use it'* last bit of life to drive it to a shop, IF you can safely do so, as in, backroads or somewhere that if it gives completely, you won't get slammed by another vehicle.
FUN FACT - "rebuild in a bottle" type products are useless.
HOW much will this cost - When talking to a tranny shop, ask these questions -
"How much will a soft rebuild cost, assuming there are no bad hard parts?"
"Do you rebuild the whole thing, or just replace the bad parts?"
Stay away if they just rig the thing. There are a LOT of small parts in GM trannys and things that simply MUST be gone over. Have an idea of the cost, cause they may charge $300 just to remove it and tear it down. At that point, a rebuld is crucial, cause during teardown, other things tend to get ruined if they are baked together.