Each dragon eats different amounts but 20-60 crickets a day is a good average range.
The first 4 months of a bearded dragons life is typically when they eat the most crickets, as much as 100 a day. However most newly bought dragons are around the 6-8 week age (except for petstores, they tend to buy them much longer) and a good 6" in length. These need to still eat at least two meals a day of insects (crickets, roaches, etc) and still can wolf down 20-60 daily. A single dragon really isn't that expensive to feed. Picking up a 1000 crickets every couple weeks at about $20 (once you add shipping etc, but could be less or more, depending on which place you get them from)isn't really expensive.
Once your dragon reaches 8 months, he won't be eating as much crickets and greens will make up half or so of his diet. So within a year, you will be spending money on greens which can still cost $20 a month, depending on how much he eats and how much goes bad and the cost of greens in your area.
Overall, bearded dragons are not that expensive to keep compared to say dogs or cats. UVB bulbs will add $20-$40 every 6-9 months and vitamins/calcium powders are good for a couple years before needing to be replaced. Vet bills will be the most expensive part, should your dragon get sick but vet costs is a consideration for all sorts of pets, be it a dog needing its yearly shots, or a bearded dragon needing a yearly or every 6 month fecal test.
The cost of feeding bearded dragons realy makes an impact if you breed them. In a year, a single female bearded dragon, mated once with a male, can produce 1-3 (or as many as 4-6) clutches, each containing 10-20 eggs. Thus, three clutches of say, 15 eggs, will mean 45 babies, each of which can eat as much of say, 50 crickets each a day. Do the math. It does add up to as much as $25 a day easily just on crickets. Hence why I always emphasize be sure you have money to spare before venturing into breeding bearded dragons, or any other animal for that matter. Unless breeding truly spectacular dragons, it may be very difficult to sell all the babies or even to make a profit, much less cover costs, especially in the first year.
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PHLdyPayne