Sounds very much like full sale price.
I think Washington took umbrage with the way Tesla was defining vehicle base prices last time around.
All model 3s were sold as "$35,000 base price + $9,000 Long Range Battery accessory + $5,000 AWD accessory + $5,000 Performance Package + $5,000 Premium Package" So for the purposes of "Base Price" every single Model 3 regardless of trim was a "$35k" Model 3.
I still take umbrage with Washington trying to charge extra sales Tax for a battery which isn't a luxury item though. Tesla's pricing breakdown was far more fair. I think it's fair for someone who buys the AWD or Performance package to pay more in RTA taxes, but I don't think it's fair for someone who pays $10,000 for a battery vs a $100 for a larger gas tank should pay taxes on $9k more car.
If you buy a $50k Model 3 you're paying $5k in taxes even though a $35k BMW M3 is comparable in luxury and features. A $35k Bolt is equivalent in luxury to a $17k Chevy Sonic. They should have just exempted $20k in "EV Equipment costs" and let you spend as much as you wanted.