NASA's total budget is approximately one half of one percent (0.5%) of the federal budget. Even at its peak in the 1960s, it never reached 5%. Seems pretty reasonable.
Every year tax payers specify where their tax money goes. 50% of their taxes will go directly where they decreed it be. The other 50% and any debt can be allocated by the congress.
There should also be options of "Let congress handle it" and "whatever the previous years makeup was" and options to adjust from there.
NASA will probably be better funded, by a huge margin.
This is why we elect representatives. It’s not a perfect system but the average citizen would not have the context necessary to make direct decisions on it.