The PetCo sale for the tank only - you will need to purchase everything else: substrate, decor, filter, heater, lights, hood if you want one. Don't forget to factor in the cost of extras that might not be included even in the 5 gallon complete deal - food, water conditioner, live plant fertilizer. These are all things you will need. If you aren't able to make frequent trips to the petstore to have them test your water, you might want to invest in a freshwater test kit.
A 5 gallon tank will probably cost less than $100 when all is said and done. If all you want to put in it is a betta, you don't even really need the filter, you will just have to resign yourself to doing frequent (2-3x weekly) water changes Filters can be very hard on bettas as they don't handle current well. Ebay will be your best friend if you can get your parents to help you - inexpensive heaters (you'll want probably 25Watt heater in a 5gal), AND aquatic plants. It is not hard at all to plant them, you just will want to read about planting requirements for any plants you get.
As Byron said, you need to think about what you would really like to put in this tank. If a betta is what you want, a 5 gallon tank is perfect. If you'd like a variety of other fish, a larger tank will be necessary.