If you are not using iTunes Match, then any music you play on the iPad in iTunes is local. No data plan needed or used.
If you use iTunes Match (costs money), music may or may not be local depending on how you synced things. If it is not, then it will be downloaded (use your data plan if it applies) the first time you play it and cached. Subsequent playing will not use data plan unless it is removed automatically; either because you have not played it in a long time, or your iPad is running low on space.
By combining iTunes Match with normal iTunes syncing you can sync some songs so that they are always available locally and leave the bulk of your (less loved) songs in the cloud.