clean the wound everyday with some warm water, and watch it.
getting your rat to the vet is the wisest option, but please take good care of her and keep the wound clean, if you do so and it is only a cyst it might heal without getting infected. Watch it evolve, if it gets bigger or stays red and ugly or with pus, get her to the vet ASAP, that's your only option anyway, no matter the price.