Okay - first off those who are think that a license or permit should be need to keep big sharks - here's a few facts to consider about how hard it is even with these permits or license.
For example - lets take a young Lemon Shark.
18' ft diameter pool (minimum for a juvenile Lemon) - about $2,000-2,500
EDPM 45 mil fish safe liner - about $500
Marine salt for 7,600 gallons - about $1,200
Filtration - at least $10,000-15,000
Sand (~1,000 lbs) - about $800
Live Rock (~1,000 lbs) - about $3,000
So it at least 17-23K before adding the shark. And it would likely be more - if you add things such as heaters, chillers, and other equipment. Also a juvenile lemon shark will be at least $1,000(USD). Juvenile Bull sharks usually at least $6,000(USD). And then on top of all that - you have to add food, vitamin & mineral supplements and basic meds needed to maintain the shark.
So basically keeping a requiem shark (i.e BTR, WTR, Bull, Lemon or similar shark) is going to be fairly expensive from the very beginning. Even smaller requiems like Sharpnose or Bonnetheads really need a pool that at least 18' in diameter as juveniles. So no requiem shark in cheap, to keep or maintain. And keep in mind these are just to keep a juvenile Requiem shark in for a few years. To maintain a requiem shark for life ultimately requires a swimming pool sized pond or lagoon that - ranges from about 15,000 gallons (Bonnethead & Sharpnose) - nearly 100,000 gallons (ie Bull Shark)
By comparison - most juvenile benthic species can spend at least the first 2 years -possible for the shark's entire life (depending on the species) in a 1,000 gallon pond.
Pond - 10' in diameter - about $1,000
EDPM 45 mil fish safe liner - about $200
Marine salt for 1,200 gallons - about $200
Filtration - at least $3,500-5,000
Sand (~300 lbs) - about $240
Live Rock (~300 lbs) - about $900
So it costs about $6,000-$7,500 to set for a good sized pond for benthic sharks. And most of the popular benthic sharks can be found for under $500 - some for even less than $100.
But at the get go - Benthic sharks are about 3 times less expensive than a requiem to set up for.