Depends on where you’re starting from. From my town, it’s about $8.50 each way to/from Penn Station, and it’s usually a 35-40m ride (edit: assuming NJTransit is on time, lol), with roughly hourly trains on weekdays and every 2h (plus a transfer) on weekends.
If you’re starting from down in (e.g.) Princeton, though, it’s going to be more like $19.
Yes, but it’s not a huge savings. 10% on a weekly pass (so only worth if if you’re coming in all 5 days) and maybe 25% for a monthly (assuming you’re in 20 days in the month).