Poly house cots for 1 Gunta(33ftX33ft i.e. 1 acre=40 Gunta) = Aprox 1.25 lacs. If you go for more than 1 acre, it will be 36Lacs/acre. This cost dosenot include the land cost.
Since you are beginner you must attain atleast 1 year of experince in polyhouse farming before doing it on your own. If want to learn on your own farm you will pay premium fees for your lessons.
You can avail subsidy from NHM. I hope you are aware what to grow. You must grow Exotic vegetables like Zucchini,Lettuce,Celery, colored capsicum and all vegetables must be sold at premium prices otherwise you cannot make profits.
The issue with starting up small poly house is, you must depend on a middle man who buys at farmgate. If you have your own logistics, it costs you more. i.e. ratio of cost of tranport v/s your produce. If you have bigger volumes the cost will increase minimum.
So before venturing into anything, Please identify your buyer and cost. Work out economics of logistics, labour availability in your locality etc.