how can I keep my lawn green at low cost ?

It depends what you mean by 'lawn'.

A picture-perfect grass lawn is a wonderful thing. However, you have to understand that it is profoundly unnatural and an inherently unstable ecosystem; it will fall apart if you stop giving it attention. As orksecurity said, to maintain your lawn as perfect green grass carpet against the onslaught of nature takes time and money. The fact that it requires so much effort is part of its appeal as a status symbol.

Imagine you live out in some wild woods. You keep this colony of pampered little chihuahuas in your yard because you think they are so cute and love showing them off to your neighbors. In the woods nearby, there are roving packs of ferocious, intelligent, and very hungry wolves. Now suppose you go off on vacation for a while and forget to ensure that your dogs are fed, the fences are maintained, and the wolves regularly culled. So... when you come back, is your yard going to contain A) cute lapdogs or B) well-fed wolves?

Pampered chihuahuas might be cute, but they take work. But if your goal is merely to keep the wild wolves on your property happy, you have chosen a much easier task.

In the same way, your lawn will only look like a golf course for as long as you put the effort in. But if you choose to cover your lawn with plants that are well-suited to your climate and will out-compete any weed, your job becomes much easier.

Are you willing to consider alternatives to grass?

For example, red clover is an excellent choice for many climates; just read this glowing description of its qualities.

I would suggest aeration to help reduce compaction in the soil, give the roots room to grow, the water a better chance to absorb, let the little microbes in the soil break down the dead grass underneath, get air into the soil, the whole circle of life thing. Those aeration shoes aren't that expensive, and all you have to do is stomp the yard once a year. Also, water early in the day, before it gets hot. The soil will absorb the water and not be so thirsty when it gets hot later in the day. As far as fertilizer, just sprinkle a little coffee and compost on the lawn. It already has it's green and brown to make nutrients, just help it a little. Grass is pretty tough like that.

First, think about what you mean by a "lawn". Are you one of those who needs a perfect expanse of completely unvarying green -- or do you want ground cover that will prevent erosion and be comfortable to throw a blanket down on -- or do you just want attractive ground cover? The answer will make a large difference in what you want to do. The remainder of this is pure, uninformed opinion. Feel free to ignore it. I grew up in a house which was defined by that last one: the front "lawn" was all pachysandra with a few flower plantings, rather than grass. Very little care needed. My house now is operating on the middle definition. My back "lawn" is a random mix of ground cover, including grass, violets, crabgrass, some dandelions, and I'm not sure what else.... and the back corner is being filled in by English ivy, which is more in the pachysandra range. Does it look like a Better Homes And Gargoyles lawn? Not really. But it serves all the purposes I want a lawn to serve, and again it requires essentially no care beyond mowing it.. I do use a mulching mower, and I compost, so I'm not paying for fertilizer... but so far I've had better things to do with the compost than feed the lawn. If a spot starts looking bare -- which the strip by the curb has been -- I'll loosen it, throw down some grass seed and fertilizer, water for a few weeks, and not worry too much about the fact that I'm going to get a bunch of other volunteers along with the grass. If you're in the first group... well, you're working against nature rather than with it, and that means you're going to have to accept higher costs than _I_ would want to pay, either in time or money or both.