I too would advise against implementing your own 1553!!!! (think 1.21 gigawatts in 1955) It is a tough call to build what you can buy. Also, Condor is not the only company; see NI's site for some of the other companies that make 1553 cards.
The higher cost 1553 cards are for testing your code and running various simulations. A production card is in the lower end and will cover most applications.
Also, I think the high cost of the cards may be that there is a lower sales volume compared to say the run of the mill serial buses that we more commonly use.
Also, glad to help; I have gotten a lot out of reading these forums and from the OpenG stuff; feels good to reciprocate.