Hi there,
I recently kicked off into this journey myself. I mainly went for it as most of the local breeders of feeder rodents offer utterly bad quality and are drastically over-priced as they base their prices on competitive cost including shipping (and still paying taxes). I've considered ordering in some bulk frozen, but at this point I didn't much care for storing all the frozen stuff (and having to buy a 2nd freezer). So I went to buy rats.
My initial startup cost was around $150 all said and done. This was:
(1) 50g Tub
(1) Roll Hardware Cloth
(1) 100pk Zipties
(1) Water Bottle
(1) 25lbs Bag of Mazuri 6F
(1) Brick of Shredded Pine Bedding
(1) Jclip Pliers
(1) Bag JClips
(1) Small Wire Cutters
(1) Large Rodent Wheel
Some of this stuff was more nice to have, or was buying the right tool for the Job.
I've now spent around $30 on Rats. I bought 1.3 and the first male I got was a biter (wasn't supposed to be) so I killed him off and replaced him recently with a nicer male.
At this point, I'm about 1 month into having Rats and I have (4) pinkies that are 6 days old to show for it. So right now, each rat pup has cost me around $45...
I need to buy a grow out tub soon, so that will be another $25, and Hardware Cloth which will be another $15. So $220 and I haven't gotten a Rat that is big enough to feed to a Ball Python.
Now, to answer your question about monthly cost once I'm up and running.
It's taken (4) Adult Rats about a month to eat through 2lbs of 6F and I keep their food hopper full at all times. So a 25lb bag appears to last 1 year per 4 rats at this time.
I change my bedding completely every other week in the tub. I spent $6 on 34lbs of pine shavings and have barely dented it. I suspect that this will last me a year as well for (2) tubs.
Monthly cost? About $4 to keep my (4) rats. It may be closer to $10 when I get a 2nd tub going.
Now, this doesn't include all the little "Extras" that don't cost me anything really. If I eat an apple, the rats get apple. If I'm cooking with fresh veggies, they all get whatever is going in the stew pot. I also supplement their 6F with small amounts of my premium dog food, which is negligible.
Hope it helps.
Paul