Reloading 9mm is not a very profitable thing to do - by itself. However, if you've already got the press and related equipment, the only extra cost involved is the dies. Those generally cost around $20-$25, depending on where you get them, and cheaper if you get them used. By the way, it takes way longer to make this work if you've got to buy the brass - and I'll assume that anyone who's going to reload 9s already has a bunch of it.
For me, the costs are approximately as follows, per 100 (and it helps to buy in quantities of 1,000): 115 grain FMJs will cost in the range of $4.50-$5.00. The powder will cost about $1.00-$1.50, and the primers will cost something like $2.00. Thus, excluding brass and your labor, the cost to reload 100 rounds of 9mm is $7.50-$8.50. Let's use the mid-point of $8.00, and assume that you're saving $1.25/100. No, that's not a lot at all - but if you appreciate the non-quantifiable benefits of reloading (quiet time, relaxation, etc.), then the time spent doing this is well spent. Strictly from a cost perspective, saving $1.25/100 will pay you back for your dies in well under 2,000 rounds, and after that you help to pay off your press, scale, calipers, etc.
I reload 9s because I like reloading them, and I like the idea of being able to make my own ammo for every caliber that I own (except .22LR, but everyone should have a few thousand of those) in the event of a disaster. The cost savings and the accuracy benefits of reloading really apply in spades to other calibers (mainly rifles for me, though also for .45s and some defensive .38 rounds as well).