Ender 3 upgrade

My 13 year old just bought I is first 3d printer from Facebook Market Place. saved hus allowance money and got it for $50. not the best in the world and extremely slow, but good enough for him to play with to see if he likes it before spending more money on a better one. it is extremely hard to level the bed and keep it that way. what would it take and est cost to upgrade to a BLtouch? it's also running 1.2.1 firmware.