Cost on co2 tank is going to depend on where they get co2. Some places you just give a deposit, and they swap tanks instead of refilling the same one over and over again.
OP, kits to do this are available (e.g., https://www.kegconnection.com/fizz-kits-carbonating-kits.html). The kits seem to leave out a carbonation stone, which i think is a really useful thing to have. Just shoving a bunch of co2 into the headspace will eventually carbonate something, but not that quickly. A carbonation stone makes the co2 diffuse faster (similar to the way that soda stream sprays bubbles).
Honestly, it's not that complicated, and if you go to a homebrew store, they'll probably get you up and running for cheaper, and they'll be happy to help. Regulators and other parts have really come down in price over the last 10-15 years.
You could go drink by drink or do a whole keg (they make really small ones now).