Hiya,
Yes it is definitely worth sending players to TC up to the age of 23. As for cost just try putting a player in a camp and when you get to the stage before you click to send them it will ask 'how many weeks' put 5 and it will show you how much it would cost, then either send him or go back to 'training home to check on another player. Always only send players for the full 5 weeks to get the full benefit out of any TC. Check out
http://www.mentaldog.com/mz/coaches_and_training.htm and also the rest of the site as it is full or useful information and tips.
Also I have dug up these three old thread about TC's and will be worth a read.
http://www.managerzone.com/?p=forum&sub=topic&sport=soccer&forum_id=12&topic_id=9216080#34009678
http://www.managerzone.com/?p=forum&sub=topic&sport=soccer&forum_id=12&topic_id=9203292#33967891
http://www.managerzone.com/?p=forum&sub=topic&sport=soccer&forum_id=12&topic_id=9187996#33916028
good luck
John