i understand ur pain cuz i expeience tank leaks before.
Frankly, no point trying to silicon back the glass. . might as well invest in a new tanks.
3 footer about $800-$1000
4 footer about $1200-$1500
5 footer about $1800 above
6 footer about $2400 above
price differ from place to place and types of glass u use for tank. . say sapphire front panel etc. .