is it worth while to have frozen semon taken on an old stud dog before he dies. is it worth the price?

It would depend on how old he is and if he has enough semen and it is healthy enough to freeze. When a dog gets some age on him his semen count will usually go done and alot of times it will not survie the freeze.

tell me more is it something most vets do or do you got to find a specail vet?

One of the leading vets in the country is Dr. Pew in Orem Utah phone number is 801-255-5395 Last time I had a dog collected it was about $350 and about $270 each time after that. it is about $80 a year to keep it, no limit on that dog.
The first time they collect they will run tests on it and see how good the semen is and then they will freeze it and then bring it back out and see if it makes it. So you might loose up to half of your first collection in tests.