Cost will varry depending on where and what type feeder insects you use. Beardies under a year old will eat TONS of insects!
Our local pet stores sell crickets for $.12 each... when our first beardie was a juvi it was much cheaper to purchase 1000 crickets from an online dealer...the cost was about $15 for a months supply...our guy averaged 30-50 a day. Even with die off and the ones that we had to throw out because they got too big for him it was still much cheaper this way!
I'm now breeding my own silkworms. I spend about $10-15 a month on chow. Most pet stores do not sell silkworms...
Salad (veggies) is another expense. Depending on what your family uses I'd budget around $2 a week for veggies.
A fecal runs around $20 average in my area. A vet check and fecal cost me $40.
If you set aside $25 a month you will have plenty of money for food and some to set aside for vet care as needed.
HTH
DM