http://www.thefa.com/thefacup/more/prize-fund
Both cups are financially pretty much worthless. It's just the "prestige" of winning the tin pot.
As I've argued 10x over on here, I'd take a 4th place finish ahead of winning either cup five times personally, as 4th gives you the potential to completely overhaul the level the club competes at for years, whereas the FA Cup in the modern era does precisely nothing long term. See Wigan for details.
......I dare say there are commercial benefits that come with a cup win (merchandise, increased sponsorship etc) but it's not a question of either/or, we should be striving for cup success and top4.