Like others have mentioned before, the cost of the fish depends on the show quality. Veiltails are the cheapest around my place at $4 and the half-moon tails cost around $10. You can get more expensive ones from breeders.
I set up a Betta tank just today and I can list my expenses.
IKEA side table - $15. (Used an old table and didn't buy a aquarium specific one which could cost around $30)
10 gallon Aquarium kit from walmart - $30 (tank, filter, hood and lights)
50 watt heater - $10 from Amazon.
20 gallon air pump - $10 from Amazon. $5 air stone.
10 lbs gravel - $10
Live plants - $5
Bought a male and a female for $9 each.
Had to build a separator to host both fish in same tank without killing each other. Cost around $5 and one hour. Lots of videos on YouTube. Can buy a ready made one from pet store for about $15.
Total amount for setup $110.
I'm using water treatment chemicals and pellet from another aquarium. Else they'll set you back about $30.
I'd say a $150 for a first time set up with a decent size tank if you buy everything new. Used items from Craigslist and such will cost significantly les. Or you can buy one of those 1 gallon 'Betta Cubes' with air pump/filter with hood and light, all for $20. The other expenses will be maintenance chemicals and food.