The difficulty of adding plumbing is one of the reasons that additions of baths or kitchens can be the most expensive types of additions. Its impossible to advise specifically without knowing more about the house, but you must tie into existing plumbing at some point. That can be near existing fixtures, or where the sewer lateral leaves the house (usually towards the street). As you can guess, this is a major draw-back to a construction.
The best thing to help save money and plan the project is to obtain house plans with the plumbing rough-in. If you must tie into a line in the interior of the house, the concrete can be cut with specialized tools, but must be done by someone that understands foundations, slabs and plumbing. This is not likely a DIY venture. There are more things than sewer pipes buried in that slab (post tensions, water pipes, power).
Be prepared to pull permits for this job for your own protection (budget fees). Obtaining permits and using licensed contractors is be best assurance that the job will be done right, you are protected against damages, and can claim the increased value for the structure at sale time. Finally, even though you paid a lot of money for this house, with prices continuing to climb in your area at over 20%/year, it will not take long for you to have more than enough equity to justify the addition and costs. If you need to defer the project for financial reasons (you are maxed out), it will be easily eligible for loans, and will seem more affordable within a year or two.