The simple answer to this question is depends on your eating habits. I am a Lacto-Ovo- Vegetarian, what it means is I eat- Dairy, Eggs and Vegetable based products only and no meats, fish etc.., because of this I cannot say how much it will cost you if you eat meat or fish on a daily basis.
Over the past 2 years, we have not noticed a substantial rise in our monthly grocery bills. I feel one of the reasons for this is that our restaurant trips have reduced to once or twice a month. All the money I save from a restaurant trip, which will serve me greasy food, I blow it on Organic food, which will be healthy and fun for me. ;-)
I know it may not seem possible for all of us to completely stop eating outside due to our busy life-style but believe me it is not impossible. There are a lot of good, healthy recipes which don't take more than 20-30 mins and for recipes which take longer time, try to use pre-cut or frozen ingredients (although buying pre-cut means, getting reduced nutritional value). Some of the other tricks involve buying canned beans without preservatives or chemical in it, making recipes which use quick stir-fry rather than slow-cook method.
For the last 2 years, I have tried lots of new recipes, some of which I improvised some of which I gave up after the first try, just because they were not worth the efforts. There are a few recipes posted on Indian Recipe Club which are quick and healthy. Most of my recipes involve fresh ingredients but you may substitute frozen, canned or pre-cut as per your convenience.
Last but not least, there are days when you just feel tired or lazy and you don't want to do anything in the kitchen, for these days make a list of fast food joints (Eg- SubWay) or restaurants which serve comparatively less greasy and healthy food and enjoy your day off ;-)