Are taxes really so high in poland that my net will be only 3500zloty if gross is 5500?
You could reduce the tax burden by asking your potential employer to employ you as a one-person company. That way you pay 19% flat rate tax (less deductions for things like telephone and internet bills, part of your rent, computer purchases, etc) and the social insurance contribution is taken down to about 200zl per month (but only for the first two years).
Another option is to simply be employed on an umowa o dzielo (a contract for a specific task). With that you pay no social insurance contributions at all but you also have no access at all to the Polish health care system (unless you pay for it).