The 19-year-old, who signed for Stoke for £3.3million last month and was loaned back to Birmingham, was in superb form as he constantly denied the Addicks before finally being beaten by Yann Kermorgant's 88th-minute header.
The home side thought that they had won the game until Wade Elliott pounced deep into injury time to extend the Blues' unbeaten run to five game.
"As Tony Pulis says, 'I have got the bargain of the century'," said Clark.
"He is an exceptional talent, one of the best young talents in Europe and every day he improves as he has got that desire and determination. How many young lads would have turned down the opportunity to go and play for Chelsea?
"He looked at the football side of it and he wants to play and educate himself and couldn't see himself in their first team for a while yet. That for me shows you what the lad is about, that's why he is going to be a top keeper.
"It won't be long before Jack is pushing Joe Hart to be the number one - and I think Joe knows that.
"We are really pleased with a point and it shows the character of the group as this isn't the first time we have scored a late goal to take something from the game."
Butland saved efforts from Ricardo Fuller, Johnnie Jackson and Lawrie Wilson in the first half before twice denying Scott Wagstaff after the break with superb agility.
The draw leaves Birmingham in 16th place with 38 points from 31 games.