our TY has a base cost on of 200e (give or take a bit) which covers school application (120e) art supplies, folders, local trips, buses, printouts, usbs etc. if you wish to do retreats or courses, they cost more (like ours is an extra 300e) but they are optional. there are maybe one or two books, so it's cheap enough.
you may find it hard to make friends in 5th year, if most people are doing TY. TY can help confidence, and some subjects are started in TY for LC (like our school does irish oral practice). 5th year is generally a basic recap of important coursework from TY, but as most pupils have done it, you may find it hard to catch up.