When you say cheap ecig, are you wanting to try it for the first time and see if it's a fit for you? Is the form factor (the same shape and size of a regular cigarette) important to you?
How much do you smoke and for how long?
You can get a little package for as little as $10 that comes with one small ecig and two 'filters' called cartomizers that are pre-filled and ready to vape. This would be a short-term solution.
A bigger and better option is going to cost around $70 to $80 for everything you need to start your vaping journey.