KEEPING KAREN PERRY working isn't cheap. Karen Perry is a 29-year-old welfare mother of four whose story was told on the front page of this paper last Friday. She is determined to keep working at her low-wage job -- at least as long as she, and the taxpayers who help to make it possible for her to work, feel they can afford it. The question -- and it is at the heart of much of the argument over the cost and reward of social programs -- is this: Is it more profitable for everyone concerned if Karen Perry doesn't work?

From her point of view, the answer in stark monetary terms is obvious. Working is expensive. After payroll taxes and the reductions in her welfare and food stamp benefits caused by her earnings, she nets about $40 a week from her work -- less if transportation and other work costs are counted. Working also puts a lot of added pressure on the time she has available for her children, housework and other concerns. So the administration's proposals for reducing government benefits to the not-so-truly needy -- the poor-plus-one, as we call them -- may well tip the balance in favor of Karen Perry's staying home. Reductions in federal support for Medicaid, food stamps, day care and other social mean that local governments will have to eliminate some servces or start charging fees to users with modest earnings. And the welfare grant that supplements Karen Perry's earnings could also be reduced by proposed restrictions in income deductions for working recipients.

Having Karen Perry stay home may not look like a bad financial deal for the taxpayer either. Her current job is not a budget bargain. Savings of about $350 a month in potential welfare and food stamp costs to support her are more than offset by the expense of keeping two of ther children in day care centers. And Medicaid still covers all the family's medical expenses. Her salary as a receptionist at a Head Start center is paid by the government as well. True, Head Start is widely regarded as a worthwile program, so presumably her services are worth her pay in the job she has. But here she may be atypical: the jobs held by many other similar workers, which are currently funded through CETA and other service programs, have been judged "make-work" by the administration, which holds that the funds could be better spent elsewhere.

So why not encourage the people in this particular set of circumstances to stay home? The answer is that when you count up the real costs of such a policy, you find that everyone loses.

When Karen Peery works, she builds a better, more dignified life for herself and her children. And, if all goes well, the taxpayer will get his money back, and then some, a few years hence. If Karen Perry has no more children -- and the odds of having more children are much lower for women who work than for those who stay at home -- the costs of providing day care will disappear in a couple of years when the children reach school age. Meanwhile, they are learning things in ways that she would have a hard time duplicating on her own, even if she had nothing else to do. As her earnings increase, as they are likely to do if she works steadily, her welfare and food stamp benefits will gradually diminish and finally disappear entirely. In a permanent job, employer medical insurance benefits will also replace Medicaid coverage.

Even more important than these direct savings, it seems to us, is how Karen Perry and her children regard the society and their part in it. She is proud of working, and the model she provides to her children will surely influence their own feelings about work, self-worth and self-reliance. If someone had offered Karen Perry a job back when she was a teen-ager, she might never have ended up on welfare at all. But, at age 29, she and her children are still a very good investment for the taxpayer. The wise and civilized society will give up a lot of other subsidies before it gives up on Karen Perry.