This is a estimated guess of how many bugs a bearded dragon can eat in a month. 
This can give you a better understanding of how costly they can be at younger ages. As a first reptile it's always best to go for an older one as they are more forgiving than the fragile babies, and a lot less costly. 
Depending on where you get the food from it can cost more than others.

1-4 Months - 
10 bugs a feed is 630 bugs in a month. 
20 bugs a feed is 1,260 bugs in a month. 


4-12 Months -
10 bugs a feed is 420 bugs in a month.
20 bugs a feed is 840 bugs in a month. 

12-20 Months -
10 bugs a feed is 210 bugs in a month.
20 bugs a feed is 420 bugs in a month.

20+ Months -
10 bugs a feed is 120 bugs in a month.
20 bugs a feed is 240 bugs in a month.