any idea how much a tank refill is?
You can buy an electric air pump to recharge the canister.
I don't have one of these air rifles, so I'm not the expert. But I looked up a dive shop in my area and they charge $9 to refill a paintball or air gun tank, $12 for a larger tank.
They also charge $9 to fill a scuba tank, but you must have a valid dive card in order to have a scuba tank filled.
You can get high pressure tanks that are the same volume as a scuba, and I am guessing those are the $12 refill.
Now, your next question will be "how many rifle refills per scuba tank?" and my answer is "I dont' know."