It really depends on three things; your currency (Im going to assume US dollars), the size of the aquarium, and the quality of heater you wish.
A decent quality heater for a moderately sized aquarium (20-30 gallons) will be appoximately $20-25 USD.
In general heaters cost around $15-40 USD depending on the variables discussed above.
Check out the heater section at www.bigalsonline.com And have a look at the Jager Heater line. I've attached a link.
A heater can raise the temperature of a fish tank if needed
One can purchase a fish tank heater at various pet supplies shops. PetSmart, Petco, Marine Land, and Argos all have fish tank heaters for sale on their websites.
a big one!
Circulation
If you have a heater, turn it off.
FIlter, heater, tank
mine cost £70 pounds and it came with light, filter and heater
Usually there is a heater that warms the tank on purpose.
Most fish are around 78 degrees F.
The only thing that you can put in a tank that small is a betta fish or african dwarf frog. Tropical fish do better in a tank that is ten gallons or more. If the tank has a filter and heater you could put one guppy into it but that would be all and I doubt it has a filter and a heater.
It all depends on how much the tank holds and the wattage of your heater I like to have 10 wattage for ever gallon so if you hVe a ten gallon tan you need a 100 watt heater also the temperature of you want will make a deference normally my 20 gallon tank can go from 46F to about 77 F in two hours
you can put in sea snails and more brightly fish that you cant get with a heater in your tank