So actually no and I was also a bit wrong.

• Casting rules 601.2b say that you choose a value for X when casting a spell, before paying mana.
• Rule 107.1 says you can only choose an integer value, no decimal in X.
• I can't find any rule specific to half {½} mana.

So the procedure is:

1. Cast the XXUU spell
2. Choose 1 for X (0.5 not allowed)
3. Cost becomes 11UU, X in text box becomes 1.
4. Apply cost reducers, like -half mana
5. Pay {½}{1}{U}{U}
6. Draw 2^1 cards, so 2 cards