The cost of employment depends mostly on what kind of agreement your worker will be employed. Different will be the cost of employing an employee under a contract of employment (umowa o pracę), and another when you hire him under mandatory contract (umowa zlecenie) or contract to perform a specified task (umowa o dzieło).
In the case of the employment contract, the net salary accounts for only about 60 percent of the total costs. From the gross wage deducted are: insurance contributions and income tax. Presently the minimum wage in Poland is 1750 PLN gross (1286,16 net)
A detailed list of all costs for 2015 you will find here:
Earning the minimum rate, the worker gets 1286,16 PLN (on hand), and the total cost of employer is 2112,96 PLN.
In addition to costs such as wages, taxes or social contributions, the employer shall bear of course the costs associated with initial medical examination and periodic employee health and safety training, equipment and maintenance of the workplace (eg. office equipment, purchase computer software or professional literature) and ensuring him (in some workplaces) workwear and beverages as well as the necessary tools for the job.
Other contracts (umowa o dzieło/zlecenie):
Using the same example, where the worker gets about 1286 PLN net - your total contribution would be from 1500 PLN up to 1770 (depends of the kind of work, kind of employee and the place of work). So as you see your cost would be lower.
Obviously the possibility of using different kind of work agreement is regulated by Polish law, however many companies are hired basing on those other contracts, and order the kind of work which should be under an employment contract.
Therefore, if you include in the contract-order, that the place of the order execution is your company and it will be done under your supervision, then it certainly may be subject to accusations that it is a contract of employment.