i don't know, it might, but i believe neutering is more effective in countering the aggression of males.
you could probably do it anyways, if only for the health benefits! (no uterine tumors or pyometra!)
i think it would cost around $100, but i've never had it done so i'm not sure.
Spaying has great health benefits at any age, and is proven to reduce the risk of mammary tumors as well as increase life expectancy. It won't do a great deal for agression issues, certainly not to the same degree that a neuter would improve a male.
Costs depend greatly on where you live. I know people who's vets will do them for as low as $60, but in my area you're looking at close to $300.
I'm a huge advocate of neutering and spaying I have 13 rats, and all are speutered. Personally, I've never had an aggressive female, but I've seen quite a few bullyish girls calm down after they're spayed and their cagemates are altered also. Plus, female rats are very, very prone to mammary tumors (something like 70-85% chance of getting them), and spaying greatly reduces that. As Jellybean mentioned, it would also stop any chances of pyometra (infected uterus, fills with pus and blood) - which is very prominent in intact female rats.
I pay $60 for a spay, but that's on the very low end of normal - spays are usually anywhere from $80-$175 depending on the vet, and the area you live in. You have to find an experienced, rat-savvy vet for the surgery.
Anytime any animal is put under there is a risk. Risk can be greatly reduced by precautions beforehand. Do not put a rat under anesthesia if it seems ill at all or if you do have your vet run blood work beforehand. It probably is a good idea to run blood work before any surgery actually, especially if it's an elective procedure.
But the best possible thing you could do is to go to a vet that comes recommended by a fellow rat owner/rescue and has done hundreds of spays and neuters. Risk is increased in rats by inexperiance and misconceptions so please do your research.
Questions I would potentially ask my vet before a spay/neuter/surgery:
What anaesthesia do you use?
How many rats have you spayed/neutered? What is the success rate in these surgeries?
Are you comfortable working surgically on such a small delicate animal? Do you work with any rat rescues/breeders I may contact as a reference?
Do you recommend antibiotics and pain killers after surgery? (if the vet says no, run run run! These are a must IMO)
What kind of stitches do you use?
Do you have follow up checks on spays/neuters?
And then it's good to ask how much, and if follow up checks, pain killers and antibiotics are included so you aren't surprised with any additional costs.