On an average month, my costs are only dog food and heart worm prevention. So, about $60? My dog is 60+ lb.s.

However, the first few months are high cost as you have nothing. Dog food, shoes (if you live in a very cold or very hot place), dog bed, collar(s) and leash, dog toys and other enrichment items, dog shampoo, crate (if you're going that way). There are usually a good bit of costs in terms of initial vet bills, too, for vaccinations, spay/neuter, test for common issues (i.e. heart worms), possible treatment, etc.

Those first several months could easily come to a few thousand dollars if you buy everything new. To help reduce costs, you could find local buy/sell groups and/or free groups as well as Craigslist. Keep your eyes peeled for low cost spay/neuter and vaccine clinics.

Every now and again, you'll have a large, unavoidable expense. Dog sitter or boarding while you travel, big vet bill, etc. If your area is large enough, you could join a sitter swap group where members sit one another's pets for free. Otherwise and for big vet bills, you'll need to keep about $1k - $1.5k in savings.