$300 a month for just essentials seems pretty high to me. Other then in the beginning for all their puppy shots they don't have to go to the vet monthly. We have 3 and we spend maybe around $30 for dog food and another $10-15 for treats. $20 for nail trimming. Grooming we do ourselves, and a bottle of shampoo/conditioner last at least 6-8 months, so that works out to only a few dollars a month. Heart worm medicine is monthly, but what's that like, $30 for a six month supply? That works out to only around $85 a month for 3 Yorkies.
Now, toys, clothes, car seats, bags/carriers, collars/leashes (momma has a collar fetish
) those are were the true expense comes in. But that is controllable in so much as it is more of a want thing then a need.
Of course, if you can budget and afford $300 then go for it. If you don't spend it all put it in savings for the medical emergency times that always do seem to pop up.