ATM puts and calls are expensive and the premiums we pay are large.For buying ATM calls and puts (if they are of same expiry and same strike price it is called a straddle) you have to consider volatility in the market....the implied volatility....larger the volatility means more expensive the straddle buying is.But if one is expecting a large move in one direction,then buying a straddle is correct inspite of large premiums....
If you buy a straddle and market moves by small amount say 50 points in Nifty,if it goes up,the calls will appreciate by about Rs 25 and puts will go down by 25 and the time decay acts on both so you really make nothing but loose because of time decay....so unless you get a move of over 100-150 points in one direction straddle buying is a loosing proposition.
You get standard option calculators where you feed the variables like volatility,risk free rate of return,number of days till expiry,strike price etc and you get the correct value of the premiums....Many brokers sites have these calculators. Sharekhan has one on its site.
If large move is not expected and implied volatility is high,then it is better to sell the straddle and pocket part of large premium. But then one is exposed to high risk and it is like a "chakravyuha" one needs to know how to get out of it if things turn sour....recommended only for experienced traders with deep pockets....
If the expected move takes time to come through, then straddle buyer has disadvantage of time decay of both contracts working against him.
Hope I have answered your querry.
Best Wishes,Trade well...and safe...
Smart_trade