There is no official disclosure on the amount of money that has been budgeted by Larian to develop and produce BG3.
Based on my understanding (and assumptions) from what i have seen previously with other games i think that BG3 would have a $100 mil + budget easily.
It is public knowledge that Larian have (they have confirmed this) over 300 staff working across 6 international studios.
Say if we only say 4 years of development 300 staff on say an average of $70,000 per year is $21,000,000 and times by four this is $84,000,000. Then you have other development costs plus after release care of the game will easily add up to 100 mil +.
I honestly think BG3 is going to be on another level epic. The only issue i currently see that might de-rail it is if Larian over do the wokeness for example the last star wars movies had big budgets but the extreme wokeness completely killed the franchise, user reviews where average 4 out of 10 across meta-critic ect...
Fingers crossed this game will be done right and the woke crap will be toned down.