Considering just the snake - not all the equipment required to keep it initially - the prices are highly variable and depend on the rarity of the snake (including the color form you choose), whether it is captive bred or wild caught, whether it's easy to breed and the size of the clutch of eggs (or live young) produced after mating.
An additional factor is where you buy your snake from.
The cheapest is generally a wild caught, common snake bought from a specialist dealer, or perhaps a commonly-available captive bred specimen direct from the breeder.
Corn Snakes - US prices: $20 - 50, UK prices: £20 - 40
King Snakes - US prices: $25 - 80, UK prices: £20 - 40
Garter Snakes - US prices: $5 - 15, UK prices: £20 - 30
Milk Snakes - US prices: $40 - 100, UK prices: £35 - 50
Rough Green Snakes - US prices: $5 - 15, UK prices: £20 - 30
As for the supplies you'll need such as a cage, heater, water bowl, food etc. you can easily spend $100 - 200 in getting set up though you can save money by either looking for complete setups in your local paper or buying off eBay.
Often, snake keepers will easily sell their snake, but the cage and equipment sits around unsold.
Paying attention to the classified ads in your local newspaper is probably one of the cheapest ways of getting set up, but make sure you check all electrics before handing over money as once a heater or light is broken, the chances of repairing it are very slim, and you'll normally have to go out and buy brand new replacements.