I do not know navigation gps in Sri lanka. but in general
(1) you do need to pay monthly bills. One time payment to buy equipment. Updates are now life time free (check with them).
(2) If you have GPS in your smartphone, you may find good gps applications (free+paid). Check whether they work in sri lanka and also their quality. Some dedicated gps providers also has android or ios applications (e.g. Garmin).
(3) I am using Copilet software with tablet (in Canada) . It works fine. I just paid around 10$ for north america maps. that is the only cost