I ran a quick test and discovered this calculated field will work
IF [Value] = "NA" THEN 0
ELSE INT( [Value] )
However, it's HIGHLY recommended that you do this outside of Tableau. If you have 100 calculated fields running this IF statement for 400k rows, you will likely suffer a performance hit.
Hope this helps,
FYI, if you use a Tableau data extract, there is no performance hit for the IF statement as written by Brad because Tableau will materialize the results of that calculation inside Tableau.
However, with 100 initial measures + 100 calculated fields for the measures + dimensions + other calculated fields there could still be a performance hit because Tableau prefers "tall" data to "wide" data. (Tableau can have some performance limitations as the data sources increase in complexity). In that case, you might look into either creating multiple Tableau data sources (each with a subset of the measures for different analyses) and/or reshaping the data so there are more rows and fewer columns.