I don't think "realistic" really needs to be, or is, a factor in budgeting. For obvious reasons I won't bother to point out.
I think, and it's just my opinion, that budget categories are different and probably should be treated that way. It still needs to be "fair" and it shouldn't take away from gameplay. You have 6 budget categories where 18m is 18m. My 18m is the same as your 18m. However, you have 3 budget categories that are far different. I can spend my 18m poorly, and get poor results, and you can spend your 18m wisely with much better results. To me, pretending that all 9 categories are the same is silly.
But, again, there's no real need to change it. But change would make perfect sense.